some of them are mining already have millions of Tbtc4
Testnet4 is at block #25039 and even if one person had mined all the blocks they would have 1.2 million coins not "millions of tbtc4"! It doesn't matter though, testnet coins have no value and also most probably it will reset when things are decided.
https://mempool.space/testnet4Tbtc3 will be online always, there is no possible way to shut down a blockchain with POW, many people will use Tbtc3 due huge distribution for more than a decade.
Maybe. But don't forget that we aren't talking about any blockchain. We are talking about a TEST network. When the test environment becomes useless, it will be abandoned sooner or later.
Also have you noticed it is the version 3 already? That means we've already shut down 2 of them successfully...