I have a 15x20ish room as well. As my consumption is almost entirely HT and TV, I went with bigger bookshelves versus towers. If you listen to plenty of music, towers will probably be the way to go, particularly if you're going to wait to upgrade the sub.
For a room that size, the 4T should probably suit you nicely. I would definitely jump up to the 5C for your center, though. It matches well with the 4Ts, and for that size of a room, it should be a great fit.
If you can swing it, the 5T is probably the best bang-for-the-buck option for that size of a room, particularly for music, but your mileage may vary. You may find that without a sub, the 4T lacks a little on the low end.
Since you have little speakers currently, and you're going to piece your system together, I'd go in this order, personally:
L/R Towers
Center
Sub
Surrounds
And obviously, the more you can afford faster, the better in creating your system.
A couple of final things to keep in mind: A) 30 day in-home trial, so you have 30 days to determine if something is enough/too much, B) They have a 1-year trade-up policy, so even if you live with it for a while, you can always upgrade (sub not included, so get what you want early), and C) Quality remains long after the price is forgotten. No matter what you choose, make sure you're satisfied, as only your ears can decide!
Good luck!