I got that #4 is the base carb. As such, I thought #4 has no adjustment so I don't know what you mean by "base idle screw." Here's what I know/have worked with - four idle mixture screws, three carburetor adjusting screws (one screw each on carb 1, carb 2 and carb 3), and the throttle stop screw.
The large idle stop screw in the rear center of the carbs, where the cables connect.
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