Testnet 4.

Sets up Testnet 4, a separate network for testing Bitcoin software, whose coins are meant to have no value.

Testnet 3 had become hard to use: a quirk let difficulty reset to its minimum, causing floods of blocks, and its coins were being bought and sold. Testnet 4 follows mainnet's rules, with small changes to stop these 'block storms' and timestamp tricks that lower difficulty.