For me mixed begins and ends with the skill set of the female partner. If they are playing with a mid to high level 4.0 female then they won't have to change up their play style too much. You can expect that your partner will have decent hands at net, maybe no overhead put away and a serve that doesn't leave you wide open for attack. She will be able to have decent rallys and if you are active enough at net should be able to put away a fair number of balls. On your serve you may get some help if you get a weak reply with your partner putting away a sitter. On the return of serve your partner will do fine against the woman and may get some good reflex returns off of the mans serve. If she does you must capitalize and win all the points served to you so you can break them.
Playing with a lower end 4.0 woman or as a 4.5 playing with a 3.5 woman the player will have to be more aggressive. You will need to cover most of the court as their mobility will come into play. Balls that the mid to high level 4.0 will hit at net will go right past the lower partner. Don't expect too much either at net and especially don't expect much on her serve, you may get lucky with opponents that can't hit off pace serves but they will figure out what to do on her second go around. On your serve, expect to have to win the points outright either on the serve or within the first or second shot. Don't expect your partner to get many returns back into play on the man's serve (if it is a decent 4.0). She may also have trouble with the woman's serve.