4Hrs would be way to long if all goes well.............
The time taken is a bit rubbery.
Consider...backups, the swap including dongle, the s/w load (if on disk about 10 mins) then you have to load the database, then synchronise. Thats a biggie as depending on your processoes could be as little as 5 - 10 mins, or as long as 30 mins or so.
Plan on the 4Hr window, and be happy when all goes well!!