So you mined testnet4 to BlackHatCoiner's Bitcoin address?
Check closer, who is the owner of this address:
https://mempool.space/testnet4/address/tb1qy65a0ltkaxpuwc22gx2zjehac8gpgppawrmpudIsn't spending them as easy as importing the private key in Bitcoin Core (on testnet4 of course)?
It is, but:
1. Some parties, like exchanges, often don't process them correctly, and don't associate those coins with your account.
2. You have to wait for coinbase maturity (100 blocks).