Codex4 is an Android application designed to improve vocabulary retention through an efficient learning process. It allows users to add words from any text with a single tap, making it ideal for professional, academic, or personal development. The app employs FSRS, a modern spaced-repetition technique that optimizes review timing, ensuring that users study effectively and retain information long-term.

The application treats each word as a lemma, covering various forms and conjugations naturally. With features like automatic word extraction and a context-aware lemmatizer, Codex4 handles homonyms and multi-word phrases efficiently. Users can track their progress through clear statistics, and their data remains accessible even after unsubscribing, allowing for easy export of learning materials.