Hi Chony
Do a Google for wav drum samples and you will find thousands out there (Many free) that will leave Ketron (And other hardware boards) drum samples for dead. (I assume T4 can load 24bit samples, if not make sure you select 16bit samples only)
The ultimate of course is as Diki said, use VSTi, unfortunately Yamaha arrangers are lousy controllers, (They were never designed for Pro use) and you can’t load VSTi on-board, so I am afraid your stuck with wav files.
Hope this helps

Bill