Due to the laws of electricity, the amp won't deliver as much power to those 4 ohm speakers as it would to 2 ohm speakers.

As Judge Dredd would say "Da law is da law"!

Or, to put it another way, at the same volume settting as you used to use, they won't play as loud. Initially you may have to kerfutz with the balance/fader controls a bit. Once they're set you should be good to go.

On the positive side, the amp will be less stressed and be a lot happier.