@Inkblot & CJClarke:
That's my point. From what I gather the problems with DH 5 are fundamental. A few bullet wounds and profanities really make no odds.
Not that it matters, of course:
While domestic earnings are nice, A Good Day to Die Hard is really designed to tap in to the expanding foreign market (which has been Fox's bread-and-butter in recent years). The last movie did $250 million in 2007; with its overseas setting and six years of foreign growth, it's reasonable to expect that A Good Day to Die Hard will ultimately earn at least $300 million overseas.
Given Bruce's advancing years, I imagine a DH 6 is already being outlined. Possibly on the back of a napkin.