Shower drain attach to cast iron drain in basement

I am adding a shower in a basement that has a toilet and sink drain already roughed in. I have removed concrete from shower drain to the main drain stack. The main is 4" PVC into a cast iron just below the floor level and seems to be an elbow. How should I connect the shower line into this setup? The main stack is closer to the shower drain than the toilet so it makes sense to go there with it.

drain

There is no way to tell you without seeing it. There is more to it than just installing a tee, so how you do it is more important than whether you can do it.