I guess the question is whether 4.0a is actually available *today*. My impression was that it would be available as of May 31, coinciding with the National Genealogical Society conference in Providence. In practical terms, there is little difference between today and May 31, so yes, perhaps waiting a few days for TMG v4.0a is the right thing to do.
I think the important point is to learn TMG before actually transferring your DB to it. You can do a transfer for practice--that's hightly recommended--but I think it's important to get comfortable before you lock yourself in.
I'm a type-A, do it now person... does my recommendation make sense in that context? :-)