In a purely theoretical sense, yes, all speaker cables should be the exact same length. The cables present a complex load to the circuit of amp and speaker. However, in real life, this is seldom practical and difficult to achieve. Differing lengths would be different loads to your amps and speakers, but the difference when measured technically would not be greatly different. I also suspect that it would be very difficult to hear as well.
What you should not do is coil wire that is too long. This creates greater issues, and makes the circuit even more tricky and complex. Depending on the surroundings, this can lead to audible issues.
As for running 4 pair, or even 2-2 pair in walls, this should be ok. Make certain the cables are of sufficient size as to not limit the system. In wall cables have pros and cons, and you need to weigh them before deciding.
