A conversion from .mp4 multimedia file to a .png picture is usually used to a create a video like sequence of images frames from actual video file.
A lot of users had success with FFmpeg program and using the following command lime sequence:
ffmpeg -i input.mp4 -r 25 -f image2 imagefolder\images%05d.png
You can find more info about this on some of the FFmpeg tutorial websites.
There may be other programs capable of mp4 to png conversion, however, a lot of them achieve this with poor results.