I found a sample WTV file on the web and experimented. It was a 40-second extract of a US TV program called 'House', in HD.

Using ffprobe I found it contained:
  Duration: 00:00:41.74, start: 27.330567, bitrate: 5225 kb/s
    Stream #0:0[0x19](eng): Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s
    Stream #0:1[0x1a](spa): Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
    Stream #0:2[0x1b]: Video: h264 (High), yuv420p(tv, bt709, top first), 1920x1080 [SAR 1:1 DAR 16:9], 29.97 fps, 59.94 tbr, 10000k tbn, 59.94 tbc
    Stream #0:3[0x1c]: Subtitle: eia_608
    Stream #0:4[0xffffffff]: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 200x113 [SAR 96:96 DAR 200:113], 90k tbr, 90k tbn, 90k tbc

That's some audio in English and Spanish, the h264-encoded video, some subtitles, and a mjpeg-encoded video stream: I'm not sure what that is.

If GUI converters have 'media info' pages, or produce reports you can probably find the same information.

I tried converting that to an MPEG-4 (mp4 container) with the command
ffmpeg -i filename.wtv -c:v copy filename.mp4
I think that worked - it was playable in VLC - but not as smooth as the original. The clip might have been faulty - there were a lot of errors.

The resulting mp4 file, which was about 10% smaller, contained:
  Duration: 00:00:41.78, start: 0.000000, bitrate: 4613 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 4399 kb/s, 59.94 fps, 59.94 tbr, 10000k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 341 kb/s (default)

From what I read yesterday, older Standard Definition (SD) programmes are more likely to be MPEG-2 video, rather than MPEG-4. I suspect you could still encapsulate them in an mp4 file - but that's a guess.