Could you image a 4-3 2009 Pats Defense that looked like this:
DE-Warren
DT-Raji
DT-Wilfork
DE-Seymour
OLB-Thomas
OLB-Vrabel
MLB-Mayo
CB-Hobbs
CB-Wheately, Whilite, FA, draft
SS-Merriweather
FS-FA, Draft, Rodney?
I don't think it would be all that impressive. Vrable isn't nearly fast enough to play outside in a 4-3, and Thomas is borderline. Seymour and Warren aren't as good pass rushers as the typical 4-3 DEs.
They'd be hellish against the run though.