Just tried upgrade to RC4 too (on Archos 101G8).
First downloaded file from repository using standard browser, only noticed when complete (approx 61MB) was shown as a txt file..?
Moved this to update folder anyway and file was found in UrukConfig "Force Update", only tablet then seemed to reboot into previous vanilla Archos/Droid OS (most of my apps gone, no UrukConfig etc).
If the standard browser doesn't recognise a file type, it'll change it to .txt, I've had that happed with .apk before. Download it with a computer next time. For now, reinstall Uruk 1.5 then upgrade again.