Ok I have tackled this task and absolutely mastered it!!

It is EASY AS PIE!!!!!

Basically u just copy n paste WAV samples onto a USB stick.
insert the USb into your T4.

Go to "expansion" and then select "Import".
u immediately then get taken into the WAV sample list in your expansion memory and u can easily open the contents of your USB stick and select each wav individually (or group select) and then import them into the on board memory.

Once they are in the memory u can then go to a drum kit, go into the drumkit editor and u can then map all the drum kit voices to an individual sample from your expansion memory.

I have managed to make a complete custom techno/dance kit - it does take time to perfect the mapping so your styles trigger off the correct samples (if you are using pre set styles)...

I have also got a full live stage / rock kit which is a great replacement for most of the standard acoustic drum kits.

I will take some audio samples of me playing them live and post them - but basically this small amount of work has completely changed my Tyros 4 into a complete different machine.

I did a quick google search last night and found pages and pages of all the original Roland 808 and 909 full drum samples, Korg Triton, Akai, Yamaha Motif Etc. they are all in 44khz WAV format and import directly into the T4. and each sample is less than 50kb so they are lightweight and hardly take up any memory.

Nick