Along with '7'、 '4' is one of the numbers where the KUN and ON are used pretty randomly.
Like, more than usual.
For example, when counting months, usually one uses the ONyomi, but with 4 and 7 you use the KUNyomi!
ABU: since this has the same ONyomi as 'death' (死） it is considered an unlucky number.
Onyomi
Mnemonic

SHE can fit four legs in her mouth at the same time. (FYI she is not kinky, she is a female Carcharodon carcharias)

Kunyomi
Jukugo
四月

April
★★★★★
FPKANA
四 (the numeral 4) + 月 (moon)
= 四月 (April)

四時

4 o'clock
★★★★★
四 (the numeral 4) + 時 (time)
= 四時 (4 o'clock)

四時間

4 hours' time
★★★★★
四時 (4 o'clock) + 間 (a period of time)
= 四時間 (4 hours' time)
4 hours' time. As in, this bus ride takes 4 hours.

四日

fourth day of the month
★★★★☆
FP
四 (the numeral 4) + 日 (sun, day)
= 四日 (fourth day of the month )

四角

a square
★★★☆☆
四 (the numeral 4) + 角 (horn/corner)
= 四角 (a square)

Used In