Iphone4 is connected to wifi (using Unicom card) and cannot send MMS.

Hello, MMS uses the carrier's network traffic, but imessage doesn't have this requirement, as long as it can surf the Internet. Imessage requires both the sender and the receiver to be iPhone, iPad and MAC. If you want to convert imessage into an ordinary short message, as long as you hold the short message for a long time, it will be sent as a short message, so it can be converted into an ordinary short message. In order to convert text into imessage, both parties must be iPhone, iPad, iPod, MAC and other devices, and they must be able to access the Internet before sending.