The legacy format v3 encodes the JSON with base64_urlsafe with the prefix cashuA.

The v4 format uses smaller JSON keys and adds CBOR (converts the JSON to binary) then encodes as base64_urlsafe with the prefix cashuB.

This results in smaller sized tokens.

The prefix was my suggestion to make the difference both human and machine readable, iirc.

CC nostr:nprofile1qqs9pk20ctv9srrg9vr354p03v0rrgsqkpggh2u45va77zz4mu5p6ccpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhsz9thwden5te0dehhxarj9ehhsarj9ejx2a30qyg8wumn8ghj7mn0wd68ytnddakj7xph5zr

For more info:

https://github.com/cashubtc/nuts/blob/main/00.md