How to add ssa to mp4

[ssa_newbie]

Hi all
I have a ssa subtitle file and need to add it to a high quality mp4 file so I can send it to people to watch. I've read some forum threads on the site but I don't understand the best way to do it. How can I add the subtitles? Thank you

(PS I have a program that can convert the mp4 to other formats if that helps)

[videobruger]

Use MKVMergeGUI or YAMB

[ssa_newbie]

Thank you that worked great. How do I change my .mkv file to mp4?

[Baldrick]

Mymp4boxgui.