For splash control it depends mostly on the size of the shower and how much of the floor can get wet (meaning how much of the bathroom floor is waterproofed like a shower pan). I was just in a renovation where there were no shower walls at all except the back. This doesn't address the privacy issue at all, but it was large enough that splashing wasn't an issue.

In other words, if your shower area is 36" square, the wall will need to be higher than if the shower area is 60" square for splashing.

It depends on exactly where your shower head will be and what type it will be. Our shower is also 4x5. Our half wall is just under 5ft. We have a hand shower on a bar that is on the 4 ft wall about 6-8 inches from the corner. That is the shower head I use. Since it is near the corner, most of the splashes are under 5 ft.

My husband uses the rain shower head that is on an arm centered on the 4 ft wall. I believe his splashes are lower.