4awg Al on ground bar with 6awg max holes
If I have an issue I add a chair lug and screw it directly to the buss, years ago we used to split the wires into 2 equal groups but I think they quit allowing that back in 99 maybe prior, I usually carry a few single and double lug chair Lugs with my crimps ( I carry probably 50 lbs of different sized crimps) just for this reason. I do agree with George noalox or another corrosion inhibitor is needed with aluminum wire connections (I use it on all my feeders even copper).