I'd assumed it was testnet5 because segnet (the segwit development testnet) was regarded as the 4th testnet. It's not strictly true though, as Bitcoin Unlimited has the nol testnet (NO Limit) and very probably there are or were other experimental testnets that I don't know about.
There's a bit of a history of skipping numbers like this: there was no testnet that was ever called testnet2 - the first two testnets were just called 'testnet' - but testnet3 was nonetheless so named - skipping the name testnet2 - because it was the 3rd testnet. (And testnet3 is still the main testnet for Bitcoin.)
EDIT TO ADD: Incidentally, the reason why we're on the third (mainstream) Bitcoin testnet is that people started to actually trade testnet coins as an altcoin, so the developers made good on their threat to reset the testnet (with a new genesis block) to help enforce the priciple that testnet coins should have no value.