This is just implausible. Nobody with a brain is going to throw away 4 million on the speculative hope that a prediction market price is going to magically manipulate media coverage on the election.
Given the immense sums of money, little to no accountability on how it's spent, and a finite list of traditional spending targets I expect campaigns have dabbled in all sorts of unusual approaches.
Compared to typical total advertising expenditures during a presidential election, my guess is that $4M is close to a rounding error.