Available for macOS
S4 combines your notes and flashcards into one place. Write once, review intelligently. An improved spaced repetition schedule helps you retain more with less effort.
Used by professionals, medical students, and academics.
S4 handles the structure so you can focus on learning.
Wrap text to hide it. Click to reveal. Your notes become flashcards.
Review them in context, not isolation. Works in .md, math, and code.
S4 schedules reviews at optimal intervals. Cards you struggle with appear more often. Cards you know well fade into the background. Our spaced repetition algorithm adapts to how you learn.
Connect notes with [[WikiLinks]]. See every note that references the current one. Discover unlinked
mentions and turn them into connections with one click.
Draw rectangles over diagrams, anatomy images, or maps to hide regions. During review, reveal each area one at a time.
Snip regions from PDFs with Cmd+drag. Run OCR to extract text as markdown with LaTeX. Link back to the
exact page with [[file.pdf#page=12]].
See how your notes connect in an interactive graph. Visualize the structure of what you're learning. Navigate by clicking nodes instead of searching.
S4 understands what your notes are about, not just what words they contain. Find related notes by meaning and discover connections you didn't know existed. The more you add, the more useful it becomes.
Your data stays on your machine in open formats you can read anywhere. No account required. If you leave, your data leaves with you.
S4 reads and writes Obsidian-compatible markdown. Point it at your existing vault and add spaced repetition to what you've already written. S4 is designed to be used together with existing popular markdown editors, not to replace them. No import, no lock-in, just plain markdown.
Your notes are standard .md files. Use S4 alongside Obsidian or any markdown editor. Same vault,
same files, no import needed.
Flashcard scheduling lives in a local SQLite database. Query it with standard tools, back it up however you want, export it anytime.
Everything runs on your machine. No internet connection needed to write, review, or access your notes.
Want to sync flashcard progress across devices? That's available as an upgrade. But it's optional. The core app is complete without it.
The desktop app is free and has everything you need to write, link, and review. An account adds mobile access so you can fit reviews in wherever you are: on the bus, before bed, waiting for coffee.
Install S4 on your phone or tablet as a web app. Use the time between classes, on the bus, or waiting in line. Your cards are always with you.
Create cards on your laptop, review them on your phone. Progress syncs automatically. Rate a card on one device and it updates everywhere.
Search by meaning, not just keywords. Find related notes across your entire workspace, even when the words don't match.
Choose a plan based on how many cards you sync and how much semantic search you need.
S4 is free to try. Download for macOS and see how it fits your workflow.
Download S4macOS only. Windows and Linux coming soon.