iPhone 4 T-Mobile Edge when I am covered by 3G

You need a new iPhone that supports T-Mobiles AWS spectrum.
You'll only pick up "4G" if you're in their refarmed 1900MHz spectrum, I believe they use 1700/2100MHz as their main spectrum, which isn't supported by old iPhones.

You'll have to get the T-Mobile iPhone, sorry.

 

Go to airportal.de to see the 1900MHz coverage map, ie the map that's relevant to your phone.
T-Mobile is refarming some of their 1900MHz spectrum (I believe this is the spectrum acquired from Metro PCS) to broadcast "4G" instead of Edge.

Good luck.

 

http://www.tmonews.com/2013/03/taking-another-look-at-the-current-iphone-5-and-how-it-works-on-t-mobile/

Like others have said, you will only get 3G if and only if TMobile has refarmed the 1900 band, which in my experience has poor coverage and unreliable and will fall back to edge often.

To truly get 3G (or 4g since tmobile markets it that way), you need specifically the "newly tweaked" iPhone 5 that has the 1700/2100 support since that band is more widespread and avail.