IV is probably my second favourite of the series after the first one, I really disliked the second and whilst the third one was ok it only felt like Mission Impossible during the Vatican bits.
I just watched this last night on Blu-Ray as well, and I agree with this statement. I have the first M:I on HD-DVD, and it's my favorite. It was a true spy movie through and through, and the action was all supporting the plot. Ethan had a team as well (two, actually), which is kind of the point of IMF.
The 2nd movie and the 3rd movie were the Ethan Hunt show, which took me out of it. This one showed that he needed his team, and couldn't do it all on his own.
If they ever do another sequel, though, I say they promote the Ethan Hunt character to more of a Team Leader role, and focus on a new supporting cast. Tom Cruise might not go for that, though.