I'm looking to start a small project that could use one of the new 1.4 features. I'm wondering if 1.4beta is stable enough to use without any problems, and will I be able to swap it out to 1.4 stable without much trouble?
Yes, 1.4 only expects minor modifcations until the final release. You can safely assume that a starting project should use 1.4 and won't get into trouble when starting with 1.4beta. Only old code, which would need quite some work to get it ported to 1.4 will need some consideration if staying with 1.3.1 or going for 1.4.