While that calculation is good, you're assuming that hydraulic system failures are independent events, but they're not. Most (all?) of A320 G&Y failures have been caused by inadventent PTU selection. All total hydraulic failures on commercial aircraft have been caused by structural damage. A strengthened floor would have saved THY981 (along with McDD listening to Convair/General Dynamics' concerns about the floor, not getting the FAA to change their mind about the floor strength, fitting the supporting plate to prevent the torque tube from deforming, not falsifying records &c.) The addition of a fourth hydraulic system probably wouldn't have helped. Four hydraulic systems didn't save JAL123.
Of course, the proliferation of EHAs in new designs means that ship-wide hydraulic failure won't condemn future aircraft (and the A380).