This is the first Power League title that felt a bit superfluous. It's a good, fun baseball game, to be sure, but it isn't much different from the series's previous outing. There are new pitching mechanics to master, but opposing teams pose less of a challenge than PL3's stout squads and most of the new material is of the "bells and whistles" variety.
Addressing the new stuff won't take long: there are nighttime games, animated bits on the scoreboard, and more stadiums to play in. It's all nice but kind of insignificant when there's nothing really fresh about the gameplay.
The most noticeable mechanical tweak concerns the pitching system. You aren't granted as much control over pitches as they head towards the plate--no more of that waving-them-around stuff. This new system doesn't feel great at first, but it's more realistic and requires more skill and thought than previous PL pitching systems. Pitchers seem to last longer as far as pitch counts go (perhaps to account for the inevitability of more balls being thrown), which is cool.
The game keeps track of a few more statistical categories than its predecessor. Don't be surprised if the league leader board ends up filled with your players, as PL4 is easy and, for a Power League vet, unrewarding.