Need bathroom rough-in guidance: branch squeezed b/w WC and soil stack

[PeaboKC]

Remodeling 2nd floor bathroom and pulled diy plumbing permit. I'm replacing lead drain lines to all fixtures (bathtub, lavatory, WC) to PVC.

I intended to reproduce existing design layout where the 4" line branches off to bathtub/ lavatory line (1 1/2") immediately before the water closet. I was going to use a 90 bend to the WC with a side inlet as drain line to lav/ bathtub... essentially what the old system was doing... but now understand that is not to code. Additionally, seems riskier with the new-to-me low flow toilets.

Any layout/ fittings suggestions on how to join the bathroom group connections to the existing cast iron tee fitting of the soil stack, considering how close the WC is to the stack? Help!!!

Additional notes:
- I sized up a 4" street bend to 4x4x3 combo wye to replace existing WC to Lav/Tub line connection but that setup is too long.... with those fittings WC pipe would come up at 18" away from wall. A 4x4x2 would come up 16" from wall.
-Shimming out the wall 4-6" would also extend lavatory/vanity... and it would then interfere with door.
- I already separated lead pipe/ brass collar from cast iron soil stack fitting. Planned for a Fernco donut and short pvc stub out to whatever fittings work.

 

[Terry]

You don't need 4" there. Three inch is plenty big for the toilet. You do need 2" for the lav wet venting the bathtub.
Normally I would be catching the lav and tub with a new line from below. The old plumbing that pulled off the closet bend I've never been able to make work with current fittings.