I have a concrete floor. Currently there are three toilets, and one sink. I have removed one of the toilets in an attempt to install tub/shower combo using the 4" opening the former toilet drained into. The opening of the tub drain sits in front of the former toilet opening. How can I plumb the new tub into the old toilet waste drain?

Hate to say it, but you are going to have to break up some concrete. It's the only correct way to accomplish what you want to do. The toilet drain does not have a trap, so you will have to break up concrete, reduce to correct size drain pipe (1 1/2 for tub/shower or 2" for stictly shower), install trap, and repipe to correct area for roughin of tub or shower. This can be a bit of a job, so let us know if your up to taking it on yourself and we will try to help. Sorry, there just isn't any quick and easy way to do this correctly.