Hi Toni,
Welcome to the RiffWorks forum.
The T4 software doesn't have a REX file player but in case you go for RWStd.,...
REX files are separate from RiffWorks - they're not included with the software, you have to go out and purchase/find them from suppliers like "Drums On Demand" etc. They are pre-recorded sounds (often drums/percussion but also basslines, etc.) which, when loaded in to RiffWorks, can replace or enhance the Instant Drummer feature.
The good thing about REX files is that you can have up to four loaded at one time.
I hope this helps. Read the REX Player Guide here->
http://www.sonomawireworks.com/guide/index.php/RexPlayer