hey, couldn't pm back because forum said you can't receive them yet. Here are some of your answers.
You would not have 4 mufflers but you would have four tips. See this pic of my car. You would need to mimic the rearmost pipe section and tips except with a stock muffler.
Buy the tips of your choice from the vendor Cyba on the forums. other tips will most certainly not fit the m3 rep rear bumper. Contact Gary @ Suvneer and you should only pay $280 for the bumper. make sure to tell him quad cut outs.
Also, checkout forum member johnsonz1122's set up. This is almost exactly what you would need to do with the piping. His muffler is almost identical to stock in size. He bought Cyba tips too. He has an mtech bumper and a quad diffusor. It is a little further apart then the m3 rep.