testnet4 support

[jlopp]

FYI testnet3 is being deprecated; you can find the parameters for testnet4 here: bitcoin/bitcoin#29775

[zzzhan]

Looking forward the issue

[cleot]

any update? or are we waiting for testnet5? thx!

[cakesoft-shashank]

Hi, I just wanted to kindly follow up on this issue to check if there have been any updates.

[yuvadm]

Seems like #570 is in the works, with nothing blocking it except maybe a review