Can I connect a toilet to a cast iron p trap?

I'm renovating my basement bathroom and I'm putting the toilet where the shower used to be and moving the shower to a floor drain. I busted up the concrete in both locations and both locations have 4 inch cast iron. Here is my question:

1. Can I hook my toilet up to a cast iron p trap? I know the toilet has its own p trap, but I don't want to tear out the existing one in the ground. I'll basically have 2 traps in series, which I think is bad, but how bad???

I'm using a doughnut flange to connect the existing cast iron bell to a 4 inch pvc pipe. Also is there any advantage/disadvantage to having a 4 inch toilet flange?

Thanks for the quick reply. Not what I wanted to hear. I've been lucky so far in my work and I haven't broken any of the pipes during my excavation.

You cannot put a toilet over ANY "P" trap, if you expect it to flush AT ALL. The older ones did have enough water and force to push through a trap, but none of the current ones can do it.

Thanks again, I'll dig deeper and cut the p trap off and replace with a 90 elbow. Hopefully it goes smoothly.