You can set Bitcoin Core to run in testnet4 mode by putting chain=testnet4 or testnet4=1 in the configuration, or by using equivalent command-line options (-chain=testnet4 or -testnet4).
This is contrasted with the options available for testnet3: chain=test, or chain=testnet, or testnet=1 (and equivalent command-line options -chain=test, -chain=testnet, or -testnet).












