Sewer Pipe.

Today I went and bought a bunch of sewer pipe and went through all the trouble of getting a helper. After all said and done, it was the wrong size. For some reason unknown to me I have had it in my brain that I am using a three inch sewer pipe instead of the preferred four inch. What was I thinking? Well I will find out what my thoughts stemmed from when I pull the floor under my toilet and see a three inch pipe out of necessity. What can you do?