Speakers and OHM's

Question

I have two realistic mach one speakers and the woofers are compleatly messed up and need to be replaced. On the magnet of the woofers it says 6 Ohm's, i want to know if i can replace them with a 4 Ohm woofer. By using a 4 Ohm woofer would it fry the internal crossovers, or do any damage to the tweeter or mid. or anything elce?