stratum+tcp://solo4.antpoolandfriends.com:34255
No uptime or functionality guarantees but here's a solo mining endpoint for testnet4.
In recent months I've not had any luck mining a block on testnet4 with 90 TH/s. There is someone running some kind of mining attack, I believe by manipulating the block times. Previously I was able to find blocks with the CKpool software.