Where does it say 1.8rc4 is released? AFAIK its happening on Wednesday Night (redflo time) Redflo, lives in Germany.
1.7.5 Has been released.
There is no set upgrade process between the release candidates. as they are really developers code. It can usually be done by running the upgrade script, but there is no gaurentee that it will work correctly.
The 1.8 final will upgrade from any 1.7x release.