Change backend endpoint from /v3/ to /v4/. Both instances of bch-api used by Cashtab have upgraded to the latest master from the psf repository at https://github.com/permissionless-Software-Foundation/bch-api
This version of bch-api uses /v4/ instead of /v3/ to denote some breaking changes, though it is still backwards compatible with /v3/.
This change does not impact the methods used by Cashtab currently. However, additional methods will only be available at the /v4/ endpoint.