So initially (in 2011 when DAZ Studio 4 first came out) there were three different levels of DS4: standard, advanced, and pro. Advanced had more features than standard, and pro had the advanced features plus even more. Since February 2012, the standard and advanced editions have basically been discontinued, and the pro version has been available for free. Technically it’s still possible to run the standard edition…that’s what happens if you don’t supply a pro serial number…but the pro serial number is available for free if you order DAZ Studio Pro through the store (again, it’s free).
DS4.5 is the most recent stable release of the DAZ Studio software, and DS4.6 is the most recent beta version. Both have all the pro features as long as you supply a DS4 Pro serial number.
Edit: oops, I hadn’t noticed that a new stable release came out today, and it’s DS4.6: http://www.daz3d.com/forums/viewthread/22156/