The best an amp can do is double its wattage with each halving of the load. The amplifier topology and the capacity of the transformer all work together to determine the actual wattage. From 200W into 8 ohms to 400W into 4 ohms, the current of the amp must double. Most amps are current limited as the load decreases. Most reach a point of instability as the load goes even lower (i.e. 2 or even 1 ohms). The amount of rated power at 4 ohms compared to 8 ohms gives you an idea of how much current the amp can deliver.

50% more wattage at half the load is not bad, but there are "better" amps out there. Some receivers actually decrease in wattage from 8 ohms to 4 ohms, as they are limited by current and the possibility of overheating.

If you have a low impedance speaker, you can either buy an amp that nearly doubles its power into 4 ohms or just buy an amp with lots more power than you need. That way you won't be pushing it too hard, even with a 4 ohm load.

With the 770, 300W into 4 ohms is still 300W. My amp does 125W into 8 or 200W into 4 - a little better than 50% more power into 4. I also have 4 ohm speakers. I have no problem driving them. I have driven the amp into clipping with the opening of Toy Story when the bass is really pounding and during the depth charges of U571, but never for music.

Any decent amp will drive 4 ohm rated speakers. The question is how loud do you want to be able to go. You can try the 770 and return it if it doesn't have enough volume for you. If a 200W amp doesn't cut it, then you are gonna have to pony up some serious bucks for an even bigger amp - buy the 750 for the other 5 channels and then a 250W or 300W x 2 (into 8 ohm) amp for your mains. The other option is to passively biamp you mains, which would give you higher volumes using 4 amp channels of moderate power. Are the Thiels bi-wire/amp capable Roderick?