ohms is the measure of the speakers resistance on the line.
if depends on how they are wired.
if ur speakers are connected in series one after the other, add them, then two 2-ohm speakers make 4-ohms. (as if u have one 4-ohm speaker)
if ur speakers are connected in parallel, then its calculated as:
1/R = 1/R1 + 1/R2
so two 4-ohm speakers would be as if u have one 2-ohm speaker.
if each speaker has a dedicated line to it, then u dont got an issue.
small story short, replace the head unit alone, and test the sound, if it works, it works, if it doesnt, replace the speakers too