You can very easily take the SkyTrain from T4 to the Metro Light Rail station at 44th St. and Washington. Hop on a train going westbound towards 19th Ave./Montebello, exit at 3rd St. and Washington, walk about a block South to the US Airways Center/Talking Stick Arena.

I would count on catching a cab for the reverse trip after the concert because I think you'll be time crunched. Assuming an on time start at 7:30, there will be one or two opening acts probably, that puts you at 8:30ish, another half hour or so to reset for U2 and you're looking at them taking the stage at 9.