I did receive an answer from Spiffy:
There isn’t a simple and feasible solution to adding that kind of logic to your order form.
It’s something that we could potentially help develop for you guys, but it would require our advanced custom coding package that starts at $600.
The best bet is to simply break out the Single-Pay and the 4-Pay options onto separate order forms, just like you did with the (Add/Remove) $500 deposit! We do have a code snippet that will hide the One-Pay option from a specific order form and automatically select the payment plan.
Let me know if that makes sense or if I can clarify anything.
Here to help!
this kind of helps our situation but not all the way really - no logic and lots and lots of order forms for us.