Somi is a 4th gen soloist, and about the same age as other 4th gen artists, so she sorta counts. Really, she's both gens.
Somi is a 3rd gen idol not 4th gen
she made her official debut in IOI in 2016, and then debut as a soloist in 2019
just like Minzy isn't consider a 3rd gen idol even if she made her solo debut in 3rd gen
because she debuted first in 2NE1 during 2nd gen
just like Eunbi isn't a 4th gen idol even if she debuted with Izone and as a soloist during the 4th gen
because she debuted during 3rd gen in a GG call Ye-a
and there's a lot more example like this '-'
and the age isn't an argument neither
Jisung is younger than her and have the same age than other 4th gen idol, but he's still consider as a 3rd gen idol