Hey there,
MistServer does not do transcoding out of the box and we currently do not support setting up a stream process for VoD content. That does mean options using MistServer here are quite limited.
You could first transcode the MP4 to 720p and then stream it out. I would say that this is only meaningful if you plan to stream out this same VoD file more often.
If it's a one-time deal I would honestly just use ffmpeg for this:
ffmpeg -re -i INPUT FILE -c:a copy -c:v h264 -s 1280x720 -f mpegts udp://UDPADDRESS
Something like this should do what you're looking for.
With kind regards,
Balder Viëtor
Head of Testing
MistServer