-snip-
But if I want to use my own blockhash value, what do you do? would this work?
testnet4=1
assumevalid=00000000000000000001fbcc91af1dc346662bc536cb4d8713cd992f00794970
Are you pertaining to your bitcoin.conf file?
If so, those options should work but if you want it to apply to testnet4 only, put it under [testnet4] line.
Else, mainnet and other test chains will try to read the config with hash from the wrong chain.
e.g.:
chain=testnet4
[testnet4]
assumevalid=00000000000000000001fbcc91af1dc346662bc536cb4d8713cd992f00794970