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There’s a small difference between US Letter and A4. If you have A4 in your printer but use “default” settings which would be US Letter, the printer warns you, correctly, in the same way that it should warn you if you’ve got a load of A5 in your printer but want an A4 print.
Never seen it but I hypothesise that if it were the other way round you’d see PC LOAD A4.
You’re exactly right. Those words just to you to load letter size paper in the paper cartridge (PC). You could also see PC LOAD LEGAL to load legal size paper or maybe MF LOAD LETTER to load letter size paper in the manual feeder (eg if you need to print something on letterhead).
When your printer is voiced by Samuel L. Jackson.