Hi Andrew, if you purchase v3 you would then need to purchase an upgrade license for v4. Upgrade licenses are at a reduced cost though, you don't need to pay full price again.

If you purchase V3 within a couple of months of the v4 final release, then there is an automatic upgrade available for that case but the v4 final release is more than a couple of months out right now.

There is a V4 beta release that you would be able to use until the final release though, it's available here: http://moi3d.com/beta.htm

- Michael