As CBS Sports points out above, there have been roughly ~407,000 games in MLB history - so what were the odds that it hadn't happened yet?
I simulated each game of the Padres/Rangers series (2 in Arlington, 2 in San Diego) and estimate the probability of hitting a grand slam in each game as:
Multiplying each game together results in a truly unlikely series of events:
Using this San Diego/Texas series as a proxy, there is roughly a 99.999884% chance that a four game stretch does NOT have a grand slam in each game (1 - 0.000116%).
So 99.999884% ^ 406,649 four game sequences = 62.35% - the odds that this had not happened yet. Resulting in a 37.65% chance of making it this far in to MLB history without the feat occurring.