Boris: With impedance, go over, but
never under!
I don't know any of
the math associated with impedance,
but if your receiver say 4ohms that
is just the minimum you never go
below. You can drive 12ohm speakers
with it, just might not be loud enough
for you.
My receiver can handle 8-16ohm
speakers(It's a 22 year old JVC).
I can run 8ohm and up with it,
never down.