Thanks for asking. Block/Cashapp relies on testnet3 and can't migrate to testnet4 mostly because of lack of lightning LND peers.
I don't think there are any LND Lightning peers on testnet4. We (Block/Cashapp lightning) use LDK for Lightning, but most of our peers run LND. LND support for testnet4 was just merged, so it’ll likely take time for the LND community to update node infrastructure.
We want to catch peering, business logic and lightning implementations interoperability issues in testnet. Even though LDK supports testnet4, switching now isn’t viable for us until our mainnet LND peers migrate.