Tough call if you're judging just on this season. Kawhi has missed a lot of games and he originally hurt his knee in the third game of the season ... so there's not a whole lot to go on with him. But if the Spurs are going to have a successful run in the playoffs, the answer has to be Kawhi. He fits a role next to the Big 3 that is desperately needed. If he can rebound, play D and score ~16 points per 40 minutes, that'd make him the fourth best player on the team.
If we're just talking about this season to date, Green has to be in the discussion. I know no one wants to admit he could be that high but he has played so many minutes that he can't help to but up there depending on how you judge "best".
Splitter definitely has the potential to be the fourth best. However, he'd have to start playing on a consistent level plus get more minutes. He can only control half of that equation, tbh.
Diaw has a case if we're talking straight up talent ........ but he's never going to maximize his talent due to his passiveness so that is only a theoretically observation.
I love what Jack brings to the team but I don't see the argument of him being the team's fourth best player. He doesn't really have a big enough role on either end to qualify. I think he's a very good role player and a vital piece to the puzzle ... but fourth best player on the team? If that's true, the Spurs are in trouble.
Neal? No, not enough dimensions to his game. That said, he's the only player outside of the Big Three that can somewhat consistently create for himself ... which definitely has some value.