Also isn't wizard tier 1? He shouldn't get a bonus at all, because personally that +10 to Int in the wizard is probably more dangerous than the four +10 the monk gets.
In the second part of the post I suggest Tier 1s getting nothing. And note that it's an enhancement bonus.
Classes might offer a choice of primary scores. For instance...
Fighter: Str, Dex, Con, Int, Cha; pick four
Samurai: Str, Con, Int, Wis, Cha; pick four
Swashbuckler: Str, Dex, Int, Wis, Cha; pick four
Barbarian: Str, Dex, Con, Cha; pick three
Paladin: Str, Con, Wis, Cha; pick three
Rogue: Str, Dex, Int, Cha; pick three