I'm putting in a half bath and a washing machine in our basement and I'd like to like to connect it to the main stack and drain, which is all 4-inch cast iron. The sweep where the stack meets the sewer line has no clean out and appears to have some corrosion, so I'd like to replace it. I was thinking that I would cut out the sweep about 2 feet up the stack and one foot along the sewer line with a snap cutter and replace it with PVC, making the connection to the cast iron with banded couplings both above and below ground. Above ground the PVC would include a clean out and a connector for the washer drain pipe. Below ground the sweep would include a reducing Y fitting to accept the 3-inch toilet drain pipe. I've figured out the venting and I'm planning to put in a few riser clamps to ensure the cast iron above the PVC is adequately supported.
My questions would be a) Does this seem like a sound plan? b) Are there any issues with banded couplings under ground? and c) Is there anything I'm missing?

As plumbers we almost always keep the main sewer drain lines cast iron for the entire run of the pipe... That means that here I would cut in cast iron fittings and clamp all using cast iron No hub pipe. In Massachusetts, if I cut into cast iron and then attach PVC... technically I am supposed to continue that PVC all the way out the roof.

In this case, if local code allows (most inspectors in my area won't allow... BUT a couple of them will... always a damn debate with some of them) they do make a 4"no hub x 4" PVC transition adapter (ask for it exactly as I wrote it) to transition from cast to PVC (you would need two of these for above and below)... then just use the "banded clamps" you were planning to use... Here in Mass, we primarily use the 4 band clamps... but some states allow 2 banded clamps.

Your plan seems fine.. cleanout above ground, fitting for washing machine (run 2" pipe for drain and 1.5" for vent), etc. even underground plan is good (4" x 3" PVC wye to pickup 1/2 bath is good... can wet vent toilet using lavatory vent if you pipe it 2" all the way to the vent stack). Hope this helps. If it does, please RATE my answer below! Thank you.