We had a new Web service feature in 4.5.0 that would require a significant rewrite in 5, which is why we postponed it for the beta7 release. 5.0.0 will of course take any 4.x version.
That said, it's not uncommon to occasionally have some version of a major release that cannot yet be converted to a newer major version because the minor version came out at a later date, similar to what David said.
I think 4.5.2 was released after 5.0.0-beta7. You need a project in v4.5.1 or earlier to be able to convert it.