I too have the 13 Widescreen at my office. Picked it up on sale for about $50.
That tank really isn't that much larger than a 10 gallon tank. That pretty much means you can't put much of anything in there without over-stocking it. Your existing 4 platys are already getting close to the limit.
If you follow the 1" rule, you've only got enough to add one more platy.
I stocked my tank with a school of neon tetras and a dwarf gourami.
I recently saw that they now have a 16 Widescreen.