Chony, thanks for your well 'video documented' vids of the problems you're experiencing with your Tyros4.
I definitely hear the percussion erratic volume level changes taking place with your custom style on T4, but not when loaded on your Tyros3. I assume that your custom style utilizes custom sounds (samples?) right? which specific style part(s) utilize custom UVN samples? Also, don't know if it matters, but how much (512 or 1 gig) do you have installed on your T4?
In addition to posting your vids here, I recommend you post this on the Yamaha specific user forums: PSR Tutorial-Tyros4 Forum Yamaha Personal Keyboard Owner - Tyros 4 Forum
as well, as Yamaha USA regularly monitors both here, and those forums as well, and may be able to help you. I also recommend you contact Yamaha USA Customer Support directly (714) 522-9011. Ask to speak to Steve Deming.
Good luck my friend,
Scott Scott Yee Entertainment