4K 3G Ultra firmware begins at B1.0.0. Previous firmware versions that begin with A are backwards-compatible with 2G units and only output at 1080p.
3G Ultra firmware has 3 numbers listed as BX.Y.Z that translate to:
Any firmware versions not listed below should be considered beta or unsupported and may not be available.
The firmware versions listed below will only run on 3G hardware built between 2015 and 2018. All 3G hardware can run BX.Y.Z firmware.