In this tutorial I show you how to sync reading progress between your XTEINK X4 and X3 with iPhone, iPad or any Android device running Readest. It also works with all devices that run KOReader. For example a Kindle Jailbreak or Kobo.
Breaking the Walled Garden: How to Sync Your XTEINK X4 Reading Progress With the Readest App
The E-Reader Ecosystem Problem
For the better part of two decades, the digital reading landscape has been dominated by a handful of corporate giants, each aggressively defending their walled gardens. If you buy into the Amazon Kindle ecosystem, you are largely expected to purchase your books from Amazon, read them on a Kindle device, and sync your progress using the proprietary Kindle app on your smartphone. The same goes for Kobo and Apple Books. While these ecosystems offer undeniable convenience, they fundamentally strip away the ownership and flexibility that avid readers crave. What happens when you have a massive, meticulously curated library of DRM-free EPUBs managed via Calibre? How do you maintain a seamless, cross-device reading experience without surrendering to a closed platform?
Until recently, the answer involved a frustrating series of compromises. You could manually sideload books via USB cables, painstakingly keeping track of your page numbers in a notes app, or rely on clunky third-party cloud workarounds that rarely worked as advertised. However, a quiet revolution is currently sweeping through the open-source community, driven by a new wave of hackable, ultra-portable hardware and incredibly sophisticated independent software. Today, we are going to look at the holy grail of independent digital reading: achieving perfect, bidirectional reading progress synchronization between a dedicated e-ink device and your smartphone.
The Open-Source Hardware and Software Synergy
The two heroes of this workflow are the XTEINK X4 e-reader and the Readest application. If you have been following the e-reader space, you likely already know that the XTEINK X4 is a fascinating piece of hardware. Weighing a mere 74 grams and boasting a 4.3-inch display, it is an ultra-minimalist, pocket-sized device built around the ESP32-C3 microcontroller. It is designed to live on the back of your phone via MagSafe or slip effortlessly into a shirt pocket, replacing the instinct to doomscroll with the opportunity to read. But the stock firmware, while functional, only scratches the surface of the hardware's potential.
To unlock the true power of the X4, the community developed the CrossPoint Reader firmware (and its popular forks like CrossInk). These open-source firmware replacements completely overhaul the device, offering advanced EPUB parsing, granular typography controls, bionic reading modes, and most importantly for our purposes, native network capabilities.
On the other side of the equation is Readest. Readest has emerged as a premier, cross-platform e-reading application that caters to power users. Whether you are running it on a desktop, a tablet, or an iPhone Air, Readest provides a gorgeous, highly customizable interface that respects your local files. It is the perfect modern companion to a dedicated e-ink device, but bridging the gap between the two requires a specific translation layer.
The Secret Ingredient: The KOReader Sync Protocol
How do an ESP32-based micro-reader and an iOS application communicate their exact reading positions? The answer lies in the KOReader Sync protocol. KOReader is a legendary, open-source document viewer designed for e-ink devices. Over the years, the developers built a robust, lightweight cloud synchronization server that keeps track of document hashes and reading progress percentages.
The genius of the CrossInk firmware is that it natively integrates the KOReader Sync API. Readest, in turn, also features built-in support for KOReader Sync. By leveraging this shared, open standard, we can trick these entirely different software ecosystems into talking to each other perfectly. When you finish a chapter on your XTEINK X4, it pings the KOReader server. When you open Readest on your phone, it checks that same server and jumps you right to the correct paragraph.
Step-by-Step Guide to Achieving Sync Nirvana
Setting this up requires a few minutes of configuration, but once it is established, the workflow is entirely frictionless. Here is exactly how to link your XTEINK X4 and Readest app.
Step 1: Prepare Your XTEINK X4 Firmware
First, you must ensure your XTEINK X4 is running a community firmware that supports KOReader Sync. If you are still on the stock factory firmware, you will need to flash the device. The most popular choices are the mainline CrossPoint Reader firmware or the typography-focused CrossInk fork.
Connect your X4 to your computer via USB-C.
Navigate to the official Web Flasher utility in a Chromium-based browser.
Follow the prompts to flash your preferred community firmware.
Once rebooted, connect the X4 to your local Wi-Fi network via the device settings.
Step 2: Create a KOReader Sync Account
You will need an account on the public KOReader Sync server (or your own self-hosted instance, though the public server is perfectly fine for most users). Because this is an open-source tool, registration is incredibly minimal.
Create a simple username and password. No email verification is heavily enforced, but keep these credentials secure, as they are the key to your reading data.
Step 3: Configure the XTEINK X4
Now, tell your e-reader where to send its progress updates.
On the XTEINK X4, open the main menu and navigate to Settings.
Scroll to the System section and select KOReader Sync.
Enter the exact username and password you just created.
Enable the option for automatic syncing, which ensures the device pushes an update every time you close a book or put the device to sleep.
Step 4: Set Up Readest on Your Smartphone
With the e-ink device broadcasting its position, we now need the smartphone app to catch that signal.
Open the Readest app on your iPhone Air (or whichever platform you prefer).
Navigate to the application Settings menu.
Look for the Integrations or Sync tab.
Select the KOReader protocol option and input the identical username and password.
Step 5: The Golden Rule of File Matching
This is the single most critical step in the entire process. For the KOReader Sync protocol to know that the book on your XTEINK X4 is the exact same book on your iPhone Air, the files must be identical. The sync server uses a document hash (a mathematical fingerprint of the file) or the exact filename to match the records.
If you download an EPUB from Project Gutenberg on your phone, and then download a slightly different EPUB version of the same title on your computer and push it to the X4, the sync will fail. They must be binary matches.
The Best Practice: Use Calibre on your desktop as the single source of truth. Send the book from Calibre wirelessly to your XTEINK X4 using OPDS browser. Then, send that exact same file from Calibre to your Readest app (via OPDS browser, iCloud Drive, Dropbox, or Readest's own web import tools).
The Daily Workflow: Frictionless Reading
Once the infrastructure is laid, the daily experience feels like absolute magic. Imagine this scenario: You are sitting at a café with your morning coffee. Your phone is tucked away in your bag, and you pull out the featherlight XTEINK X4. You read for forty-five minutes on the crisp, glare-free e-ink screen. When it is time to leave, you simply click the power button to put the device to sleep. In the background, the CrossInk firmware quietly wakes up the Wi-Fi chip, pings the KOReader server with your exact paragraph location, and powers down.
Later that afternoon, you find yourself stuck in a long line at the grocery store. You don't have your e-reader handy, but you do have your iPhone Air. You unlock the phone, launch the Readest app, and open the same novel. Within a fraction of a second, the app fetches the remote progress data. A prompt asks if you would like to jump to your newest reading position. You tap "Yes," and you are instantly staring at the very next sentence you were about to read at the café.
Because the synchronization is bidirectional, any progress made on the OLED screen of your smartphone while in line will be pushed back to the cloud. When you return home and pick up the XTEINK X4 before bed, it will sync upon waking, advancing your bookmark forward. It is the ultimate continuity experience, completely divorced from corporate data silos.
Conclusion: A Glimpse into the Future of Independent Reading
The ability to sync reading progress across disparate hardware platforms and independent software applications proves that we do not have to accept the rigid limitations of closed ecosystems. The community-driven efforts behind the XTEINK X4 hardware, the CrossPoint and CrossInk firmware developers, and the robust architecture of both Readest and KOReader have created a modular, user-first reading environment.
It requires a bit more technical literacy than simply buying a Kindle and tapping "Download," but the reward is total sovereignty over your digital library. Your books remain yours. Your reading data remains yours. And your ability to choose exactly which screen you want to look at, at any given moment of the day, is finally in your own hands.
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