It depends on the plate feature depth, paper, blankets, press plate flatness and everything that how big plates you can print. My gut feeling says that 4 ton press should be fine for palm sized collagraph plates and maybe bigger, but I havent done much testing so I cannot say any specific tips. In the distant history I did lot of sheet metal forming and other stuff with small 10 ton press and I can remember that with the correct tools those small presses can do lot of work. Hard or semi-hard rubber/silicone blanket could work well in such setup, but there is risk that it will tear the paper if there are deep 3D textures on the plate.