Hey Tom,
I use projection lenses all the time for making pictures. The 4" is probably going to cover 6x6 very well, maybe 2 1/4 x 3 1/4. The ones I have vignette 4x5 rather a lot, but then, there is a beauty to that as well. The 5 1/4" should be interesting on 4x5. Some of my projection lenses are Petzvals, some are triplets with a built in "throat" in the barrel, at the middle element, acting as an aperture. I have two Leitz 2.1 cm projection lenses like that- from a combination opaque and magic lantern projector called the Epidiascope. The large (12"?) opaque projector lens on this machine is a Petzval, the magic lantern lens is a triplet! My newest favorite is a 6" SunRay, by Wollensak in black enamel and chrome. I don't know what it came from, I bought it at the Photographica show in May, but it is a sweet little Petzval - ideal for 4x5.
Have fun!