With a few minor changes Im glad to report that the EmonSD build script (alpha release) works on the Pi4 with Buster! 
I’ve created a specific branch of the build script for buster which can be found here:
https://github.com/openenergymonitor/EmonScripts/tree/buster
The main changes are:
- Support for RPI4 and debian buster · openenergymonitor/EmonScripts@22dbfac · GitHub
- specify php7.3 · openenergymonitor/EmonScripts@808dcfd · GitHub
Full details of the build script - including the current state of development can be found here:
EmonSD build script progress update and beta release
See installation readme here for full steps on how to use the build script on RPI4 Buster: https://github.com/openenergymonitor/EmonScripts/tree/buster/install