I'm in a similar position, going from 4.0 to 4.5 though. I'm tending to focus on specific things...good, cagey 4.0's love to drop shot so I'm working on improving my forward movement. Weak 2nd serves don't work, so I'm working on developing a consistent kick 2nd serve.
I'm almost there. I've had a decent record in tourneys (over .500 and one final in 5 tourneys this year) at 4.0 but I want to make at least a couple more finals and win one before moving up.
Consistency goes way up at 4.0, some of the top players don't use much pace but place the ball well and will run you ragged. You should be able to run hard for a long time, fitness is so important.