Unless you can find the GuruROM (and required adapter), the latest version is 4.15. You can get a 4.15 ROM from tjlaser on ebay or amibay, he might be on here, as well, for a reasonable price (I think I paid under $15 for mine). I'm using a 4GB CF in my GVP unit, keep partition size under 2GB, no issues. You may be able to go larger if you use SFS or PFS (or OS 3.9) instead of FFS, but may need the GuruROM for that, I'm not sure.
More info here:
http://babel.de/amiga.html