When P4 first came out we weren't using such big meshes and high res textures. As the years have gone by, things have become more detailed and demanded more from the program which is why we need P5. P4 was perfectly adequate for how things were then and P5 will be perfect for how things are now but, how things are now includes higher specification computers. P4 used to be a struggle to work with on the lower spec computers that were common when it first came out. There was a really sweet time when the app and the bog standard computer were in sync...