https://www.homedepot.com/p/Commercial-Electric-3-Speed-Antique-Brass-Fan-Switch-82365/306596870?MERCH=REC-_-pipinstock-_-203077853-_-306596870-_-N

https://www.amazon.com/Gardner-Bender-GSW-32-Electrical-Terminal/dp/B00004WLKG?th=1

How did you modify the fan you've already done? That one has a single pole/triple throw switch and powers just A/C, then just B/D, then all A/B/C/D. You've figured it out once.

So on the one you've done already:

• A/C circuit complete
• B/D circuit complete
• A/B/C/D circuit complete

What you want

• A circuit complete
• B/C circuit complete
• A/B/C/D circuit complete

There's no circuit backflow issues at all. Each leg of the switch only completes the circuit for the lights connected to it. You must be thinking every light is connected together or something.