Valkyries, whether a team runs 3-4 or 4-3 hardly matters anyway, personnel and coaching matters much more...Patriots defense sucked this year, it can only likely improve no matter what scheme they choose.
Yes, while coaching matters a lot, so does the personnel. You can't run a 3-4 defense with highly ineffectual pass-rush from your DEs and no name OLB's. It gives the opposing offense too much time, and when put against teams who can block well it can make Matt Flynn's look like Peyton Manning's. The 4-3 forces the required pass rush, while sacrificing coverage in the middle, which wasn't happening with the Patriots anyway since the Linebacking corps consists of Jarod Mayo, Jarod Mayo, and his associate: Jarod Mayo. So by this change of schemes, they aren't losing anything while gaining a lot.