First of all, dont listen to the shit above about buying an ADM or other subpar mount.
Spending 450 on a GDI is a waste of money. It is pissing money down the toilet.
I have owned the slight and the extended. I would get the Extended. You can use it perfectly with a 1-4x and if you decide to move to something with more power, you can still mount it will the ER and it will ballence the rifle well.
BUIS are pretty much obsolete at this point in the game. For a non battery powered, quality optic, you don't need them.
You could also go with a riser and rings and spend a little less money. However with a 1-4/6x, the poi shift wont be noticible enough to prevent hits on man size targets at a 5.56's effective range with a Bobro QD. It has one of if not the least ammount of POI shift as far as the QD mounts go.