How do I get 3G on my unlocked iPhone 4 on tmobile

Do you have 3G enabled? Have you updated the carrier settings? Also, do you know if you're in a refarmed T-Mobile area?

 

MyTouch4G has a different baseband then the 4 the 4 doesnt support the same 3G frequencies as your mytouch 4G

 

You can get a new iphone 5 from tmobile and get 3G everywhere or stick with your iphone 4 and get spotty 3G coverage in some places only.
Up to you.
Its a hardware limitation with your iphone 4 that's not compatible with the nonrefarmed areas or Tmobiles 3G.
That's why you're stuck with 2G Edge in most places.

 

Go to airportal.de to check if you're in t-mobiles refarmed "4G" area.
If not, you'll only get Edge bc old iPhones don't support T-Mo's AWS spectrum.

 

To add to what has already been said, if you are getting spotty coverage, that likely means that the refarm in your area is in the process of being rolled out. If you plan to stick with the 4, there is nothing you can do except wait.