Most files are already basically .mp4's, unless they're Broadcast Channels, which are going to be .mpg's. They just need to be decrypted.
Depends on your cable system. Everything I get, OTA and cable, is mpeg-2, aka mpg. None of my cable channels are mp4.
I also use kmttg. I transfer from my TiVo, which arrives as a .TiVo, which is mpeg-2 in an encryption wrapper. I decrypt the file on the fly, so generate portable .mpg files. I can then go through post processing, at which the last step is to transcode the .mpg to .mp4. This last step is a relatively slow, cpu-intensive process. To me, it's not worth it. I leave it as .mpg instead of .mp4. Pretty much everything these days can play mpeg-2. So I don't bother unless I want to resize the video, change the sample rate, etc.