[Sekrin]

Click and drag the red square (e.g. 11) and drop it on the square you want it to be (in this case, you want it to be bar 4, so drop it on bar 4). Please note that there won't be any visual indication that this is working until you drop the bar and it moves.

Please also note that there is a more recently updated version of ArkInventory on Curse - as I recall, you've been able to do this for a while, but if you have troubles it might be worth trying the update (the usual advice regarding backing up your existing copy and the SavedVariables file(s) would definitely apply here).

Edit: Oh, and you can also click the Red Square, select Action -> Move and then click the Red Square you want it to appear as (again, 4 in your example), select Action -> Move: Complete.