It’s important to understand that Ability Score Improvements are not tied to character level, but class level. This means if you’ve multiclassed to a level 2 Wizard and level 2 Monk, you won’t get an ASI for reaching level 4. Instead, you’ll have to wait until you reach level 4 in either class to receive an ASI.
It is possible to build your character in such a way to receive ASIs on consecutive levels, for example, if our multiclass build above progresses to level 3 Wizard and level 3 Monk, they could then get two ASIs in a row by progressing to level 4 in both classes over the next two level ups.