The custom G10 scales for the 4 have palm swells and are longer, where as the standard factory scales, irrespective of material, are flat.
The three custom scales have cost a little over $200.00, inclusive of postage. The 4 needs them the most, the 6 somewhat and the Izula-II not at all. I was thinking of selling them and putting the money into another knife but I have been having second thoughts.
Yes, the top screw, on the edge side of the sheath, is used to adjust retention. The one opposite it, is a spare.
I know it does not make any logical sense to get the 4 when you have a 3 but I am very certain that you will like it a lot and wouldn't be surprised if you prefer it to the 3.
Since you were the culprit who made me get the 5, I thought it was only fair I repay the compliment.

Between either of the Izulas and the Candiru, I would start with the Izulas first. If you have largish hands, the Izula-II would be the better option.