if that doesn't work out then I would just cut off the exposed ends of the four inch pipe and insert a 2 inch pipe and seal it in. Two inch seems to work well everywhere else, and I wouldn't be scared to use a sledge hammer on it either. Anyway its a lot of work and good luck dudeby gator - Deep Creek Hot Springs, Apple Valley, California
my 1st option would be to chip away with a hammer and chisel and then back fill with cement around the pipe for a seal. But, if a small bolder is in the way you don't want to chip or mess with it, try to remove it and again just backfill a larger hole now.by gator - Deep Creek Hot Springs, Apple Valley, California