my work isn't playing back properly

I first built the score I'm working on in MS3, which I much prefer to MS4, but I thought I'd try to warm up to MS4 and opened my score in it. Because I hit the save button before I closed it, now it won't open in MS3, and when I open it in MS4 and try to play it back nothing happens. It did successfully play from MS4 before.
How do I revert it back to MS3, or alternatively, how do I get it to play in MS4?