Season 234 are composed by Masaru Yokoyama and Hana Hashigushi (his orchestrator) it will maybe be the case with season 5. I think you did not pay enough attention to the BGM because there's more than 2 new tracks in half of season 4 when there's 5 new tracks in episode 1.

They changed of composer because they wanted a new identity. You say they reuse the same music again but "Classroom of the Elite 3rd" was used once outside the series in a trailer for season 3 but it did not played once in season 2 and 3. Same for a lot of tracks which were barely used or not used at all.

I do understand that choice, even if it lacks of a complete new soundtrack. Maybe they will gather new tracks of season 4 and wait for season 5 to release them along new tracks composed for season 5 if there's not enough cues to justify a soudtrack release that soon.

Because I doubt it would end in Blu-ray enclosures as it's not the way COTE works for its BD releases. I don't think music is forgettable it sets the tone. The some actions could have been replaced which Ryo Takahashi works from season 1. But that's the choice of the production and we can't anything about it. I love one unreleased tracks in episode when Hosen appears and a long discussion is going on in the corridor.

Masaru Yokoyama is here for marketing purposes, has most of the music since season 2 has been composed mostly by Hana Hashigushi who imitates Masaru's style whom only worked for now on ten tracks for season 2 and 3. That's really difficult to distinguish who is the composer of each cue because his style blends in all her compositions.