Tonally the first 3 are very different, both from each other and from the ones starting with the McQuarrie era with MI4: Rogue Nation. The series, despite being almost 30(?!?!) years running, has a lot of call backs and references, as well as a recurring cast of sidekicks, so watching them all is probably worthwhile, though you could probably realistically start with 4 and be fine. Then again, Kittridge is in Dead Reckoning and he was in the first film so I bet those first 3 will be relevant for some easter eggs at least in the final(?) film.