I am posting this resolution here because I searched here and did not find an answer.
I had a problem when upgrading my clients from FP2 to FP4 using smart upgrade. The tracking database showed that all attempted upgrades failed. I knew this to be incorrect since I could check the client PC and the version was indeed FP4. After careful scrutiny, I noticed that the new version was “Release 8.5.1FP4” notice there is NO SPACE between 8.5.1 and FP4. This is a departure from the previous Fix Packs which had a space there. I changed my Desktop Policy and the smart upgrade kit to both reflect the lack of space and now clients report a successful upgrade.
It would be nice if this version number naming scheme followed some kind of standard. 