Thanks for that explanation Terry.
God I feel dumb, check out this photo, with the first image being how it is now and the second what I plan to do. Basically, if I cut the 4" iron waste pipe I can tap into it running the new pipe (blue line) BETWEEN the joists and thereby raising it. I will remove the current stretch of pipe (red line) and cap off where it meets the toilet junction. As a bonus the new shower pipe will be closer to the main vent stack.
The pipe leading out of what will be the new p-trap is about 7" above the 4" cast iron pipe. Only remaining question is do I bring the new pipe above the 4" cast iron and drop the 2" shower pipe straight down with a sweep 90, or step it down with a couple 45's.
Reason why I'm doing this is the current p trap was in unfinished space. There will now be a shower under it. Raising it gets me about 9" of ceiling height.