Q u o t e: We looked at act length in Diablo II and came to some of the same conclusions, "They ran out of time". But when considering it further the intent of the act structure actually got shorter as you went, intentionally rocketing you toward the end of the game. It's one of those things that makes game design hard to relate. It's... it's sort of like a movie score. If done right emotional music can make a scene feel 'better' or raise whatever emotion is being portrayed, but if you start scrutinizing the individual components, or just lose the immersion, it can come off as cheesy.
Act IV is more of a finale than a full act.. short and to the point. So call it 'Finale' rather than 'Act IV'. Then people will just expect a short, action-packed area.