TIll gen 5, the accuracy of Rock Tomb was 80. So chance of missing is 20%

Chance of missing 4 times = (20/100)^4 = 0.16%

Since Gen 6, accuracy changed to 95. So chance of missing is 5%.

Chance of missing 4 times = (5/100)^4 = 0.000625%

Hope this helps :)