While it can be advantageous to have identical or very similar speakers all-around, the room EQ software provided in most modern receivers minimized this requirement. To the extent that it is successful in "flattening" the frequency responses in all of the speakers, it makes them all sound very similar. Personally, when I've upgraded speakers, I've usually replaced the fronts by higher quality speakers from a different manufacturer and repurposed the older speakers as surrounds.
Most non-flagship AVRs have a little trouble driving 4 Ohm speakers to maximum volume. Their amps can't provide adequate power and often will go into protection mode when turned up too far. Bi-amping can't really help with this, since most of the power is required by the woofers. The higher frequency drivers really don't need much power to produce high sound levels.
If that turns out to be a problem, often one can use external 4-Ohm capable amps to offload the main speakers from the receiver. In other words, you probably should consider getting a model of receiver which includes preamp outputs. Getting a preamp/processor plus external amps to begin with is another, but more expensive, option.
