Unless I'm mistaken, it would be necessary to grind all Merkel trees in each mined block and find roots that collide in the last 4 bytes.

The results would be probabilistic rather than definitive, but I think it would be interesting to know something like "X percent of blocks mined by the network except for miner B contain a potential Merkel root collision, and Y percent of blocks mined by miner B contain a potential collision." Especially if Y >>X, and especially if blocks mined by B tend to have fewer transactions (minimising the search space).

Is the size of the search space too large to make this kind of analysis prohibitively expensive?