I would have preferred "Mission: Impossible IV" as well.
Part of the problem is people are starting to look down upon "numbered sequels" because so many of them have been crap, no matter the franchise. So you end up with the numbering being abandoned like Trek did, or you get these subtitled titles to try and set them apart from what came before (IIRC Mission Impossible III didn't exactly set the world on fire).
Personally, I have no interest in the franchise. I was excited when the first movie was announced but then it ended up being a Tom Cruise movie, ignoring the fact what made Mission Impossible work was the fact it was a team effort with no one character or actor dominating. And the whole bit about Phelps being a traitor (Peter Graves reportedly was offered the part, saw that, and told them to f-off) was a real middle finger to the fans. Granted, it didn't matter because most its target audience had no idea there was ever a TV show (as I found out when I started looking for the TV series DVDs at the shops and kept being offered the movies, followed by blank stares when I said, no, the TV series.).
Although I don't plan to double-dip on the 7 box sets I've purchased over the last 5 years, maybe the release of MI4 will spark a Blu-Ray reissue of the original series. The prints (from the evidence of the DVD sets) look great and would make the transition well, I think.