Yeah, 4 and 5 haven't been released yet. 4 is in development, see development thread here on it:
http://xmoonproductions.org/viewtopic.php?f=44&t=4719The listings for versions 4 and 5 in the account manager are just placeholders. The latest version is 3.5, so if you have that, you're caught up.
There is no current release date for v4, it will be done when it's done. They've been a bit quiet on the development process but xpadmin confirmed last month that they are indeed still working on version 4 and that they will release it when it's ready. A lot of improvements are coming with that version when it releases, so although it may be a long time yet, keep checking back.
I'd recommend checking out the Modding subforum here to add to your XStoryPlayer experience and help tide you over until version 4 is released. Modding subforum here:
viewforum.php?f=27Regarding mods, check out Dungeon Ex, Love Party, and Eskarn's Night Out. They all add a whole bunch of new content and stuff to play with. There's also a small mod to the Tentacle Dreams story (Tentacle Dreams Ex) which adds the ability to touch the girls before you're "supposed to" and a few other things. The original version of Tentacle Dreams Ex was only available for XSP version 3.0 (which is now out of date), but someone tweaked it to work with 3.5. Check the later pages in that thread, I think around page 10 or so is when he posted that update.
Good luck!