If you can use mkv instead of mp4 as your container format, there is widespread player and muxer support for various subtitle formats in matroska. (widespread throughout free / open source software. I can't comment on other players.)
Then you can use SRT, ASS, and many other text subtitle formats. You can even use bitmap sub formats like VOBSUB (dvd) or PGS (blu-ray).
wikipedia has an interesting table of subtitle formats.
To properly answer your question, though, I just tried muxing SRT subs into an mp4. It doesn't work.
ffmpeg -i in.mkv -i in.srt -c copy /var/tmp/out.mkv # works
ffmpeg -i in.mkv -i in.srt -c copy /var/tmp/out.mp4 # just timestamps
The mp4 has a
mov_text stream, but displaying it in mpv just shows "00:00:00.000" or some other timestamp, with no text. Ok, apparently ffmpeg doesn't get it right unless you specifically use
-c:s mov_text. If you use
-map 0 -map 1, I get
Stream #1:0 -> #0:2 (subrip (srt) -> ? (?))
Encoder (codec none) not found for output stream #0:2
(My subs are in a separate file, not muxed in the mkv, so I need a -map for each to make sure I take all streams from all files. Otherwise ffmpeg just takes the first a, v, and sub stream it finds. And without
-map 1, it doesn't try to use the srt input at all, for some reason. Like I said, it makes a mov_text stream, though.)
Anyway, I left in my mis-steps in case they're helpful when you're trying to figure out this stuff yourself. This works and plays with subs in mpv:
ffmpeg -i in.mkv -i in.srt -codec copy -c:s mov_text /var/tmp/out.mp4
The key line in the output is:
Stream #1:0 -> #0:2 (subrip (srt) -> mov_text (native))
For editting your subtitles, you can use anything that can produce a subtitle format that ffmpeg can read. (It should be able to convert any text-based format to mov_text.) Subtitle Edit is one of the newer better-maintained GUIs, and runs on Windows and Linux at least. IDK about OS X.