mp4 to png

It sounds like you want to pull a single frame from a video. You can do this in AME by choosing the PNG format, clicking on the Preset to open the "Export Settings" dialog, and navigating to the frame you want to export. Then, in the "Video" tab, uncheck the "Export As Sequence" checkbox, click "OK, then you can start the queue and only export the one frame.

Alternatively, you can bring a video into Photoshop, open the "Timeline" panel from the "Window" menu, scrub to the frame you want, and save a PNG there too.