And logically (reductio ad absurdum) if 4e and 3x get seperate forums, so should Exalted, Vampire, Werewolf, Mage, Dying Earth, Esoterrorists, Call of Cthulthu, C&C, Mutants and Masterminds, Runequest, Conan, Iron Heroes, Arcana Unearthed etc. That obviously is not a good or workable idea.
So why just 4e and 3x? Because there is some friction there? Because 4e and 3x players are incapable of inhabiting the same forum? I see no evidence of that to such a degree as might warrant DMZs to seperate the sides.
Seperate 4e and 3x forums is a very bad idea all the way around.