start4.elf not compatible on Raspberry Pi 4 rev 1.5 board
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starbuck2015
found fix for it
https://github.com/raspberrypi/firmware/tree/master/boot
I copied just these files fixup4.dat & start4.elf from link above
over from latest raspbian to mycroft firmware folder & it booted fine
got the error on below firmware
mycroft-bigscreen-rpi4-20201126-arm64-BETA2.img.gz