I am Curious about bitcoin testnet. Since there is a massive change in bicoin mainnet i.e. https://bitcoin.org/en/alert/2017-07-12-potential-split

The change on mainnet does not effect testnet at all. The mainnet change is to activate segwit. However testnet has had segwit for the past year and a couple months.

Will Testnet4 be there and Testnet3 completely wipe out?

No.

I just want to perform mining on Testnet. As some people are mining with ASIC even on the testnet and increased the difficulty. If so happens then it would be great to have 0 difficulty again.

There is no such thing as 0 difficulty. The minimum difficulty is 1. Testnet3 has provisions to ensure that it is always mineable by someone. If a block is not found within 20 minutes, the difficulty will drop to 1.