Just wanted to add some final information to this.
With regards as to whether or not the Radeon HD 4000 series supports the Media Transfer Protocol:
The Media Transfer Protocol is supported by all Radeon cards since the Radeon X1300.
The MTP, specifically in the case of "drag-and-drop transcoding", is supported by all Radeon cards, starting with the Radeon X1xxx series up through the current Radeon HD 7xxx series.
With regards to Accelerated Video Transcoding and whether or not it affects video playback:
Simply, no. Accelerated Video Transcoding specifically pertains to the GPU's ability to encode and transcode media.
These answers came from a correspondence between AMD and myself.