[ddouglass]

Theoretically yes you can install 3.321A on the new computer. I say that because there is always the chance of unforseen problems. Make sure you have the new computer up to date with all the Custom updates (except Hardware updates as these tend to be out of date anyway or not the right updates) from Microsoft before you install it. That should eliminate any problems.
Yes the 4.08 upgrade cost of $69.99. In your case 2 upgrades, one for each license of 2 installs for the 4 computers. You can either open Hoster and go to Help/Check for Upgrades and buy them through there (only buy once for each license) or go to Support/Version Upgrades and buy them through there.
I would highly suggest that you SAVE the download instead of clicking on RUN. That way you only have to download it once, copy it to a flash drive or CD and use it on all 4 computers, putting in the correct license number for each. This also gives you a back-up inc ase you need to re-install it for some reason at a later date.