Is pre-image attack useful for mining?
If a miner normally has X hashrate so it takes Y time to find a block, and that miner is able to find a preimage attack that results in him finding block in Y/100 time, then it would be as if he had 100X hashrate. So the attacker would be able to do whatever anyone else with 100X hashrate can do. If this is more than 51%, then the attacker can modify the blockchain history and everything else that can be done with >51% of the hashrate.


  Is it easier to find pre-image for hash that has to start with 4 zeroes than excatly the same hash?


In terms of a preimage attack, it depends on the hash algorithm.

Via brute force, it is easier because there are more possible values that you can get a preimage for rather than just one specific value.