Release 3.4.0 beta 4

This release brings bug fixes and minor feature improvements. See https://github.com/Duet3D/RepRapFirmware/wiki/Changelog-RRF-3.x-Beta-&-RC#reprapfirmware-340beta4 for details.

Users running Duet hardware in standalone mode can upgrade using the files here. Users with attached SBC, please upgrade via the unstable feed on the Duet3D package server.