One mistake to be wary of is the attitude that I see a lot around the club that the 3.0 defers every ball to the 4.0 unless it's right to him or in his alley. You still have to play as a team. If you're both at the net and it's right down the middle, yes the 4.0 should take that one - but if the 4.0 is serving (and coming in behind it) and the 3.0 is at the net, the 3.0 should be looking to take anything that comes across the middle because he still has better position to do so.
Other common mistake you see is either player trying to do too much. The 4.0 may start overhitting to compensate for the weaker 3.0 player, or the 3.0 player may try to play up to the level of the 4.0 player. Sometimes this happens simultaneously.