MMS service might not properly work on jailbroken iPhone 4 using T-Mobile carrier. To fix the this MMS service on T-Mobile, you need to patch up your jailbreak with the new fix that is available via Cydia. Here are the details:

This tutorial assumes that your iPhone is jailbroken already, since you are using T-Mobile. If not then follow the instructions to see how to Jailbreak and Unlock your iPhone on iOS 4.0 and 4.0.1

• Launch Cydia on your iPhone. Tap on “Manage” and then select “Sources” from there.
• Now tap on “Edit” and then on “Add”. It will ask for a path to get a source. Type in “cydia.pushfix.info” and tap on “Add Source”.
• Wait for few moments while Cydia carries out a series of installation tasks.
• When installation is complete, go back to Cydia and search for “T-Mobile US  MMS Fix for iOS4” application. Install it on your iPhone.
• Now press home button to get to your main home screen. Now go to Settings –> General –> Network –> Cellular Data Network and crosscheck your MMS settings with the following informaton:

APN: wap.voicestream.com
MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc
MMS Proxy: 216.155.165.50:8080
MMS Max Message Size: 1048576
MMS UA Prof URL: http://www.apple.com/mms/uaprof.rdf

Now all you have to do is to restart your iPhone and now the MMS service provided by T-Mobile should be working flawlessly on your iPhone.