> So basically the BIOS is incorrectly reporting
quoted text
> BIOS-e820: 0000000100000000 - 000000013c000000 (usable)

No. Count up the the sizes of the (usuable) entries. You will see
that when you provided mem=4GB, the E820 got truncated to stop at 4GB.
Without that it goes past the 4GB mark (which is correct).

Keep in mind, 4GB doesn't mean your usuable memory stops at 4GB. The
BIOS shuffles the memory around to stick in the BIOS, ACPI, PCI hole so
that part of the usable memory is above 4GB.
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