I also have a C4 PII (2013) and went from 4.1.D.R2 > 5.1.F.R4 > 5.4.B.R6 > 5.4.B.R8: works fine. (each step cost You 20 minutes)
From 4.1.B.R8 directly to 5.4.B.R8 without problems, I don't no, give it a try. I guess the system is warning you when it's not possible.
All the firmware upgrades I find in this thread and are reliable for me.
Put the firmware on a usb, formatted with FAT32, all the files in the folder SMEG_PLUS_UPG