if by any chance the search space can be narrowed down even more (for e.g. assume the first 15 chars are correct)
it can cut down the number combinations and the needed time drastically
If someone skips 4 characters while writing down a private key, chances are he also wrote down one or more characters incorrectly.
If you're sure the rest is correct, it could be worth spending some money on Cloud computing, start up enough nodes, and crunch it quickly.
Someone specialized in wallet recoveries may be able to help you with this. I'm thinking about Dave from
https://walletrecoveryservices.com/ for instance.