I agree with you and Augdog, the GEC goes to the busbar not the cabinet, but I have land #4 without an add-a-lug in a SquareD buss many a time.
Landing a #4 to the grounding buss is not the problem, as the NEC allows EGC's to be bonded to the cabinet by a screw, and allows the cabinet to act as the bond between the grounded service conductor and the grounding buss for EGC's, if we read 250.28 then 250.28(A) we will see the MBJ can be a screw, but in 250.28 it does not mention anything about a GEC, this is because its not for the GEC.
It all changes with GEC's, because of the high current nature of lightning, I guess they feel a screw is not good enough, and want a wire or buss to be the pathway, so landing a GEC to the cabinet or a separate grounding buss using a screw is a violation of both 250.24(A)(1) and 250.24(A)(4), the problem is most do not put both of these two sections together to understand this requirement.
This has been discussed as far back as I can remember on this site, but the forum change over in February 2003 will limit a search to that date, but for good reading just do a search on 250.24(A)(4) and it will give you hours of reading.
You will also see there was allot of confusion to what this code meant even back then.
If I remember right this was added to the NEC back in 1984?