Francesco, you're up against the fact that there are no shutters a really fast 4" lens will fit. To use anything faster than an f/2.8 Xenotar or Planar or Heligon you'll need a 2x3 Speed Graphic, preferably a Pacemaker. These cameras have focal plane shutters so can be used with lenses in barrel.
Here are a few candidates for you, all in barrel:
4"/2.0 Taylor Hobson Anastigmat (2 1/4" x 2 1/4") as was sold for Vinten F.95 and AGI F.134 cameras. These are uncommon and cult items so not inexpensive. I have one, it covers 2x3 wide open. SKGrimes put mine on a 2x3 Pacemaker board.
There's also a 4"/1.8 TTH for the same cameras, I've never had one and don't know whether it would cover 2x3.
4"/1.9 Dallmeyer Super Six and Rareac. Cult items and very rare. There is a 6"/1.9 Super Six, covers 4x5. I've had one. Very much a cult lens.
100/2.5 Uran-27. Very short back focus, just makes infinity on a 2x3 Pacemaker Speed when mounted in front of the board. I have one, not a bad lens but not as good as the 4"/2.0 TTH. Claimed coverage 113 mm. They turn up from time to time on eBay. SKGrimes put mine on a 2x3 Pacemaker board.
There's at least one Wild Heerbrugg 4"/1.4 Falconar on ebay now. You don't want one, covers only 6x6. You don't want a 100/1.4 Boyer Saphir or a 100/1.9 Saphir for the same reason. Boyer claims that the 110/1.9 Saphir fits a #3 shutter and will cover 2x3. Fast Saphirs are very rare and sell for silly money.
There's the 110/2 Zeiss (BRD) Planar for Hasselblad F. Very pricy and I don't know about coverage.
David, my lens diary is up on the French LF board. You may enjoy reading it. Currently in three pieces, http://www.galerie-photo.com/1-lens-6x9-dan-fromm.html
. Sooner or later in one large piece.