Personally, I would start in MVC3, and then upgrade the project to v4 when it is RTMd. Previous versions had some problems with the transition from the beta version (see below).
However, you can be sure that, as with the previous version, a quick and easy update path will be available (usually there appears a project conversion tool released at the same time).
I have a project that I'm working on right now, and if I get to the web tier before v4 is completed, I will start with v3 first.
I believe this depends on whether any of the new features, such as responsive rendering via Mobile views (or, indeed, the Web API), are an integral part of your solution. Just donโt go on the beta platform :)
July 2012 update
I ended up on the web layer of my current project before RTF RT4, so I decided to switch to the RC release first; then nuget night packages to support the web API.
Apart from the editing problems (which are documented), I did not find any problems even with the integration of the extensibility library for whitelabelling that I wrote, for MVC 1-3, which works at a very low level.
Would I move earlier if I didn't need the many benefits provided by the web API? Probably no.
But this is so, I'm glad I did :-)
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