What happens if you reference a segment greater than 4 in the ModR/M byte? For example, MOV Ew, Sw [8C] with "reg==5". Does the logic just ignore the upper bit?
On an 80186+, you'd get a #UD, but the 8086 doesn't have strictly "undefined" opcodes; everything does something.
I'm pretty sure that the extra bit will get ignored. There is internal circuitry that expands the register number in the instruction to a 5-bit register number. (You need 5 bits to handle 8-bit registers, 16-bit registers, segment registers, and internal registers.) For segment registers, it drops the third bit. But I haven't tested this case, so I can't guarantee there isn't some weird side-effect.
On an 80186+, you'd get a #UD, but the 8086 doesn't have strictly "undefined" opcodes; everything does something.