whether a given 4ohm speaker is going to pose problems will depend on:
1) how loud you listen
2) how sensitive the speaker is
3) how big your room is
(all of the above will factor into how much power you actually need to achieve a given volume)
plus as you note crossing over the bass to a subwoofer can relieve the load on the receiver's amps.
furthermore, not all speakers rated "4ohm" are the same, impedance is not a static value. There are some speakers rated 4ohm nominal which will be easier loads than another speaker rated 6ohm nominal.
so, in other words, the answer is "maybe".
but if you aren't going above -20 on the volume dial, and using a subwoofer for 80Hz and below, then chances are you will be fine.