This Video I am replaced a section of bad leaking cast iron stack from my upstairs bathroom.
Cast Iron Saw Blade: https://amzn.to/3bDDFTD
Saw: https://amzn.to/37O37VE
Epoxy Putty if you want to try and put a bandaid on your leak: https://amzn.to/3dOf1T5

4 inch PVC pipe: https://www.homedepot.com/p/JM-eagle-...
4 in. x 3 in. Fernco: https://www.homedepot.com/p/Fernco-4-...
4 in. PVC 90-Degree elbow: https://www.homedepot.com/p/NIBCO-4-i...
Remember Dnt hire a Plumber, save the cash and do it yourself.