I see no benefit to installing from ports and then upgrading. I'd simply install from a more recent source instead.
The current SVN is a further de-bugged version of 0.4-beta. I'm using that and it is quite fine. You can find a recent snapshot at
Roundcube - Nightly Builds. The 0.4 branch has many feature improvements over the 0.3.x versions.
If you have the time, you may wish to wait for 0.4-stable to be released, especially if you want to simply install it and then not mess with it for a while. Otherwise, go ahead and pull down the most recent of the nightly builds linked above.