"よねんせい" vs "よんねんせい" for "fourth-year student" in Genki 1


I've been learning japanese for a couple months and recently started to
include the Genki textbook along with my WaniKani studying. Near the beginning
of the workbook, there is a question that asks to basically describe Takeshi,
a 4th-year student, with the beginning "たけしさんは..." and the workbook answer key
says the proper answer is "よねんせいです" as opposed to what I initially thought it
would be, "よんねんせいです." Why does the word for four (よん) get shortened like this?
Obviously using kanji it would be 四年生 (which feels like it would be pronounced
in a very similar way to both the kana readings). Thank you for any help :)




The following counters make 4 read よ. They are irregular, so you have to
learn.

  * 四年【よねん】 (including -年間, -年生...; same for below)
  * 四人【よにん】
  * 四時【よじ】
  * 四次【よじ】
  * 四日【よっか】

Other ones usually takes the regular form よん, except some fixed words that
mean more than ordinary "four". Should you encounter one, you have to learn
each.

  * 四番【よばん】: (baseball) "the fourth batter; the cleanup"
  * 四字【よじ】熟語【じゅくご】: "four-kanji idiom"
  * 四畳半【よじょうはん】: "four-and-half-tatami (square) room"
  * 四限【よげん】: (student slang) "the fourth period"




The following counters make 4 read よ. They are irregular, so you have to
learn.

  * 四年【よねん】 (including -年間, -年生...; same for below)
  * 四人【よにん】
  * 四時【よじ】
  * 四次【よじ】
  * 四日【よっか】

Other ones usually takes the regular form よん, except some fixed words that
mean more than ordinary "four". Should you encounter one, you have to learn
each.

  * 四番【よばん】: (baseball) "the fourth batter; the cleanup"
  * 四字【よじ】熟語【じゅくご】: "four-kanji idiom"
  * 四畳半【よじょうはん】: "four-and-half-tatami (square) room"
  * 四限【よげん】: (student slang) "the fourth period"

