Hello looking at the wording of possession
Possession - A Robot / Disc status. A Robot is considered to be in Possession of a Disc if the Robot is carrying, holding, or controlling the movement of a Disc such that if the Robot changes direction, the Disc will move with the Robot. Therefore, pushing / plowing Discs is not considered Posses- sion; however, using concave portions of a Robot to control the movement of Discs is considered Possession.
Possession is limited to three (3) Discs. Robots may not have greater-than-momentary Pos- session of more than three (3) Discs at once. Robots in Violation of this rule must immediately stop all Robot actions except for attempting to remove the excess Disc(s).
G17 Use Discs to play the game. Discs may not be used to accomplish actions that would be otherwise illegal if they were attempted by Robot mechanisms
Looking specifically at the concave portion of the rule a robot could be possessing discs that are then controlling the movement of discs in front of them (see mage)
Treating the hands as a concave portion of a robot in this case the robot is possessing three discs while a fourth is held within a concave portion between discs, this can cause the discs to all follow the robot through turns. In this case would this then be considered possession of four discs via the combination of SG7 and G17?