I've done 500 feet of 4" duct quite often with poly twine tied to a shaped sponge between disks (5" sponge, 3 1/2" plastic disks) held together by a wire looped outside the disks. Use a shop vac to pull it through along with all the field mice that took refuge in the duct sections while they were laying in the field before being assembled and buried in the trench. Gerry :-)} London, Canada