Hi,
I tried taking a photo, but you can't really see the difference, so i just used the same photo for both.
The weaving is really close. Seems like the main different is just the Flatbottom is completely flat on one side and meshing on the other side, where P4 is meshing both sides. I can see how flatbottom might be easier to attach the flat side to an impeller. P4 is a little thicker overall though.