Hmmm, FPR draws off of 2 & 3 not 3 & 4, so I don't believe it's the FPR. And none of the vacuum lines smell of fuel that I noticed. It's probably worth a check though to pull off the vacuum line right at the FPR, as it sounds like that's the best way to check it.
Ok, so an update after some fiddling in the garage this morning: As mentioned before, when pulling vacuum hose #4 off, all cylinders fire (confirmed by grabbing headers) when just letting the hose dangle. However, when I put my thumb over the removed hose, which is clearly pulling a good vacuum, cylinder #4 will die again (appears #3 actually is firing but warms its pipe a little slower than #1 and #2). I then tried the same exercise on throttle body #1. Pull off the vacuum hose, and all cylinders fire when letting vacuum hose #1 dangle. Put my thumb on vacuum hose #1, and cylinder #4 dies again. So putting my thumb over either a disconnected #1 or #4 hose gives the same symptom of #4 not firing as if you have all hoses properly connected. I should also add that when I put my thumb over the vacuum nipple on throttle body #4 (or on #1 nipple when doing that test) the bike is unaffected at all. So in summary, cylinder #4 will fire at idle only when the vacuum system is open to atmosphere at some point, but doesnít care whether its own nipple is plugged. Iím at a loss as to what this means at the moment. No FI codes have been thrown. I guess it means itís a system problem since it didnít matter if the #4 nipple was plugged or not?