Post a picture of what you really have, and you'll get a more reliable answer. An internal flange will work on a 4" pipe. Anytime the pipe is not plumb, things get messy. In concrete, usually, you need to drill for an anchor (a lead one works, but there are other types). Sometimes, you get what you pay for...now, you've got to clean up his shoddy work. Hopefully, we can help, but we need to see what you really have first.