HI Gunnar Jonny
Not compatible with the Tyros 4, but probably convertible in Style Creator if the REX sample format could be converted to wav samples in a UVD ( Custom Drum Kit).
There is a few sample converters out there. I used Sample Converter before for some conversions in the past. If memory serves correctly, conversions were allowed in the Tyros 3 format with Sample Converter, which of course is compatible to the Tyros 4. Wish that developer could update their program to include the Tyros 4 and 5.
Haven't delved deep enough into these REX type files, but the Tyros 4 can accept the WAV/aif samples, while the Tyros 5 can accept WAV/aif, Soundfont, and now these REX files. Not sure if REX2 format works as well, but as long as Soundfont and REX can be converted to WAV samples, should be able to make them work in a Tyros 4 with some knowledge and work in Style Creator.