What is your download status?
In three weeks FC4 will be available via http, ftp, etc.
You could spend that time saving your settings and conf data so you will have it handy.
Then you could reinstall FC3, up2date everything (if your connection is dsl), leaving a primary partion for FC4.
By the time you got that ready FC4 would be ready too.
You would not have the conflict of bad packages, weird repo's, and you would have FC4 with the 200+ updates added in.
coz right now, there are lot of bugs still being fixed.
What I mean is, you can use the FC3 CD's (assuming you have those) to make a place for the real FC4 and not download FC4 cd's, not download 200 updates, and after all that end up with a flaky T3.
Personally, I would not update FC anything to a FC Test. They are apples and oranges.