Read Classroom of the Elite: Year 2 Volume 1 next, because the anime’s first three seasons covered the whole Year 1 story and Season 4 is set to continue into Year 2.

What to start with

  • Light novel: Year 2 Volume 1.
  • Manga: the Year 2 adaptation, since it continues after the Year 1 finale.

Why that’s the right point

The first 11 light novel volumes, plus volume 11.5, make up the original Year 1 arc, and the anime has already covered that material.

That means anything before Year 2 Volume 1 is basically already adapted in the anime.

Quick note

If you want the most complete experience, the light novel is the best choice, since the manga version is not as far along in English releases.

TL;DR

Start with Year 2 Volume 1.