I think this thread so far is much like the thread "C58: New try main line Two Knights Defense" in that most people seem to be largely plugging moves in to a chess engine and giving an opinion based upon what the binary demon spits out (I don't know if this is a good or a bad thing)
When I first looked at the original analysis over on googlebooks, my first thought was that 4.f4 (in answer to 3...Nxd4) seem to be a terrible weakness that, perhaps, might have been sustainable for players of that day which is why 4.Nf3 looked a considerable improvement only in comparison. Thing is though everyone is solely concentrating on 4.Nf3 Nxf3 has been largely forced when it is not. Even the original analysis is more thorough in that it examines 3...Nxd4 4.f4 Nc6 so why doesn't this apply to 4.Nf3 Nc6.? I think the purpose with Nc6 is to withdraw the knight, shore up the defences and make white prove his pawns worth of initiative...and as most lower to middle strength players are now a days seemingly (there is that word again) preprogrammed to decline unknown gambit ideas, 4.Nf3 Nc6 should be considered an opition...however in mentioning declining 4.Nf3, there is the consideration of precise move orders:
[Site "chess.net"]
[Date "2012.07.06"]
[White (me) "Mr.Wink"]
[Black "Battle_of_minds"]
1. e4 e5 2. Nc3 Nc6 3. d4 Nxd4 4. Nf3 Nc6 5. Bc4 Be7?? 6. Qd5 Nh6?? 7. Bxh6 O-O 8.Be3 d6 9. Qd2 Kh8 10. Nd5 f5 11. Nxe7 Qxe7 (+--) and eventually I drove a winning position into equal one into a lost one with 12. O-O-O?? f4! 
but that had more to do with the 9% Barvarian lager I was drinking at the time
Hadron

After 4.f4 Black can play in Falkbeer fashion with 4...d5!

Further, after 5.Qxf3 Nf6 6.Bc4, Black can consider 6...Be7. It appears to me that 7.Qg3 0-0 8.Qxe5 Re8 is unattractive for White. Nor does 8.Bh6 Ne8 look especially appealing; if 9.h4 Kh8 10.Bd2 Nd6 and soon ...f5, or 9.Be3 d6 and Black appears to be quite fine.

kylemeister wrote on 07/04/12 at 18:45:58:

Wouldn't 6.g4 be unpleasant? I mean, with Black's suffering from the half-open d-file, he can hardly hit back in the center with d5. 

On second thought, this argument is limited, because Black can play 5...d6, practically forcing a position like 5...Nf6 6.Qg3 d6 7.Be2. Would this be worth a pawn? 

In Fyfe's original treatment 4.f4 Bb4 5.Nf3 Bxc3+ 6.bxc3 Nxf3+ 7.Qxf3 (this actually comes from a consultation game in 1883), what I didn't like was 7...Qh4+ 8.g3 Qf6 (or maybe 8...Qe7).   

I suppose a plausible possibility is 4. Nf3 Nxf3+ 5. Qxf3 Nf6 6. Bc4 d6 (a Four Knights line which has been given as equal with White's d-pawn still on the board).