Much can be said on this subject. Much has been said. The bottom line is that you'll be fine connecting your 4 ohm speakers to pretty much any commercially available receiver for "home theater" applications. They are most often designed to drive 8 ohm loads, so you are correct to ask the question since a lower impedence will be asking the amp to work harder.
You can google/bing something like "connecting 4 ohm speakers to an 8 ohm receiver" and you'll get links like this:
The Tech Source
Home Theaters and more? in Southern California.
I was wondering if anyone could give me some input on whether I can purchase any receiver with the speakers I have now. They are not the greatest but my concern is they are rated at 4 ohms. I would like a sony to utilize other sony devices but am open. Just looking for good sound for movies. Will eventually replace for better speakers at a later date. thanks