Lock Cell and use a formula if another cell has value

[TJC86]

Hi,

Probably an easy one.

I have two cells: C4 & C6.

IF someone enters in either cell, I want the other cell to be locked and have a simple sum formula in (=SUM(D4,D6,D8))

So if someone enters 100 in C4 - C6 will be locked and have a Sum formula,

if someone enters in c6, c4 will have the sum formula and be locked.

Thanks!