I didn't see anything specific to the iPhone 4. All i saw was that they are now offering the iPhone (bought from them).
Edit: Just saw this on their site:
How do I update my iPhone APN settings?
If you have an AT&T compatible iPhone 3G, 3GS, 4 or 4S running iOS 4.0 or higher you may click on the link below and we can attempt to update your APN settings.
This link may not work based on limitations and configuration settings you may have on your iPhone, please contact the manufacturer or point of purchase for further support regarding your phone. Your phone must be connected to Wi-Fi in order to continue.
1 .While on Wi-Fi, open Safari and type in iapnupdate.straighttalk.com.
2. Tap Install.
3. Tap Install Now.
4. Enter your pass code if prompted.
5. Tap Done.
6. At this point you can exit your browser, turn off Wi-Fi, and re-boot your phone.
7. With Wi-Fi off, access Safari to ensure your data connection is working.
Please note that this will not enable MMS (Multi-Media Messaging) and we do not support this service for iPhone customers.
By clicking this link you are agreeing to our Terms and Conditions iapnupdate.straighttalk.com
Assuming that is correct, is their anyway to get MMS working?