There are
- 51 OLL cases with 4/216 probability
- 5 OLL cases with 2/216 probability
- 1 OLL case with 1/216 probability
PLL skip probability is 1/72
Assuming you that there is no last layer manipulation (never considering a future last layer at any point in the solve),
For 2 solves the probability of same OLL into PLL skip twice is:
(51 * (4/216)^2 + 5 * (2/216)^2 + 1 * (1/216)^2) * (1/72)^2 = roughly 0.00000346
For 4 solves the probability of same OLL 4 times is:
51 * (4/216)^4 + 5 * (2/216)^4 + 1 * (1/216)^4 = roughly 0.00000603
If assuming OLL skip is an OLL case, change those 1's to 2's.
Becomes a quite a bit more difficult to calculate when there's more solves in sequence and now you have multiple cases with correlating probability (e.g. for 5 solves you could have solves 1-4 or solves 2-5 fit the 4 solves in a row requirement).