Hello,

Right now we have a main 4" cast iron sewer pipe running under our basement floor. The very last fixture on the run is a toilet. Can I remove the toilet, replace the final 90 degree elbow with a tee and replace the toilet in the same place, then extend the line a couple feet further and plumb in a floor drain? (with p-trap of course)

Thanks in advance!

No. It will probably work but it is not legal nor good practice to leave it unvented. The toilet can pulls the water out of the floor drain trap. Not likely with 4", but could.