[Cashtab] Update api to /v4/
[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

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc