It's a generic term for all 4-speed electronically controlled automatics.
The Ford auto tranny in the 5.0 Mustangs was coded as the AOD4 (Automatic, OverDrive, 4 speed. Often referred to as the "Angel Of Death"). But it's sometimes refferred to as a 4EAT when using it in conversation. It flows better to say 4EAT Mustang, than to say AOD4 Mustang.