We've stayed at both and will be going back to POFQ in April. While the pool slide at CSR is longer and a bit more exciting, for a 4 year old, I think the Serpent slide at POFQ would be a better choice. First, it is visually appealing to little kids. Second, you can see the line for the slide and the pool part itself at the same time. When we were waiting for DD to slide down the CSR slide, we never knew how many were in line and when she might be coming down. It was a little nerve rattling.
We also like the smaller size of POFQ. While there is no sit down restaurant there, the food court is fine. I don't care for the food court arrangement at CSR. You are "served" and charged a gratuity for the help (which is really sporadic).
Good luck with your choice.