Wow, this Yamaha forum needs a boost from neglect. There are plenty of Yamaha users, but are hanging out elsewhere.
I guess a late answer is better than no response at all. The Tyros 4 does have comprehensive editing features, but for some reason Yamaha removed the more detailed drum editing.
The easiest fix is replacing the snare or hi hat sample in the Tyros 4 Voice Creator. Plenty of suitable individual drum kit wav/aif samples can be found. There is the ability to set the volume level of the inserted wav sample when assigning the sample to the corresponding note and element in Voice Creator, either on the Tyros 4 Voice Creator itself or through the included software disk that came with your Tyros 4.
My older Yamaha MU100R Tone Generator allowed individual drum kit editing, plus two drum kits could play at the same time. I also have the PLG150-DR expansion board (for the MU100R) with many more new drum kits, plus the Plug-in Board Editor software to edit the drum kits right on your computer.
No idea why Yamaha would not continue with this more in depth editor on their TOTL arrangers, but perhaps they will reintroduce some form of a comprehensive drum editor in the next Tyros model. Meanwhile search UVD format drum kits. There are dozens of free drum kits to be found, with maybe a few with a more suitable volumes on the individual drum parts you want.