Mirror Cells within the same column

[breakchad]

I need a simple formula to make two cells mirror each other within the same column. For example. I want A2 and A4 to have the same value regardless of which cell I input the data into. If I change A2 to a value of 3, then A4 should change to 3 as well and visa versa if I post into A4. Also I'm using Excel 2002. (My company is tight with upgrades lol)