It's a tough call, they are all so close that it really isn't much of a difference. For example, if they were to swap the order 17. Dirk, 18. Chuck, 19 Stockton, to 17. Stockton, 18. Chuck, 19. Dirk, you likely wouldn't have even said anything. I think the difference between 17-19 is so small that it doesn't matter too much.
Dirk has had a career that compares to those guys. Though there likely isn't many who believe Dirk at his very best was at they level they were at their very best. Knowing how much most voters for things of this nature value rings it kind of makes sense. Dirk won a ring as "his team", something none of the other 3 were able to do. KG got a ring, but wasn't finals MVP like Dirk.
And for what it's worth, Dirk is now 8th all time scoring ahead of all of them. Likely has a chance at passing Moses Malone and Dr. J this year, putting him at 6th on his career. The only guys ahead of him being Kareem, Karl, Kobe, MJ, Wilt, and only LeBron on pace to surpass him anytime soon.
His career mark is narrowly short of missing 50/40/90, with career shooting and scoring numbers that are very much comparable to Larry Bird.
My vote would probably be Dirk's career > KG's career, the other two are tougher to decide. I've always been a big Stockton fan, arguably the best point man to ever do it from an all around pure PG standpoint. Barkley and Dirk are close to me. Chuck wasn't a great defender either which was the area KG had the advantage, but he was a rebound machine. Dirk was the better shooter and therefor, IMO, a bigger threat to defenses.
I think I agree with you, that those 3 guys were better at their best than Dirk, but I might be inclined to say Dirk's career has been the best of the 4. So I guess it depends how you were casting your vote. I would probably have them all lumped together in the same tier of players, deciding a number value would be tougher. Members of the 3rd tier group.