Song files created in Studio One version 2 and version 3 can be opened in Studio One 4.
A helpful tip:
When songs created in older versions of Studio One such as version 2 or version 3 are opened in Studio One 4 for the first time, you may wish to save a new copy of these songs (e.g. if you open 'SongName.Song' >> save as 'SongName_v4.Song').
This is so that you can still open the original Song files using the version of Studio One in which they were created, which can be useful if you want to send a song originally created in Studio One 3 to a friend who does not have Studio One 4 for example.