I used to have the similar issue and here I want to share my experience.
I find a step by step guide on how to add srt to mp4 at http://www.idealshare.net/video-converter/add-subtitles-to-mp4.html
After have a try, i find it really works great and the follow features really attract me:
Add subtitles to MP4 like add SRT to MP4, add SSA to MP4, add SUB and IDX to MP4 and more;
Add subtitles to MP4 as well as keep the original video format or change MP4 to MOV, AVI, WMV, MKV, FLV, ASF, WebM, 3GP, etc;
Also can attach subtitles to almost all kinds of video clips
like MOV, AVI, WMV, MKV, FLV, ASF, AVHCD, MOD, MTS, MXF, WebM, SWF and