At a bare minimum, you need a 210mm lens for 4x5. Ideally, as Sanders mentioned, you want something in the 240 (10") to 300mm (12") range. Your Toyo Field may prove somewhat limiting in bellows extension with the 300mm for tight headshots. I would not look for a Dagor, not only because of the cost, but at 4x5, your image will be TOO sharp for a good portrait. The Commercial Ektar in 10" or 12" length would be a great lens. There is also a 12" Ektar f4.5 which makes a great portrait lens for 4x5 and 5x7, but it won't mount on your Toyo most likely, as it comes in a #5 Ilex shutter. Since at 4x5 and larger, film grain is really not an issue, shoot a faster film like HP5+, TriX or Tmax400.