According to the NYA, bit 4 was intended to be a signal for Segwit2x, and when btc1 is queried for the required softforks via GBT, btc1 has included segwit2x since bit 4 was triggered. Bit 4 is also supposed to be BIP91, so bit 4 is ambiguous. /NYA/ is unambiguous, but non-binding.
In the btc1 code the HF codepath was activated by bit 1, bit 4 was used only for BIP91.
It's worth mentioning that Segwit2x can only exist if it has a majority of the hashrate. If your definition of Bitcoin is the post-fork chain with the most hashrate, then Segwit2x will either be Bitcoin or it won't exist. It's only if your definition of Bitcoin is Bitcoin Core that Segwit2x can be an altcoin.
Segwit2x has wipeout protection for full nodes(but not SPV wallets) so technically it can be a minority fork.