[Cashtab] Update api to /v4/

Summary:
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.

Test Plan: npm test ; npm start and observe balance incl token balances are the same compared to cashtabapp.com

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8972