PS4 could have sold more if the Pro wasn't such a half hearted attempt at a mid gen upgrade. The difference in Far Cry 5 is staggering, 1620p on PS4 Pro and 4K on One X and its the same for many multiplatform games. There is no freesync and no backwards compatibility and the Switch has captured public attention. They have to do more if they want to keep the price at $300 and $400 but if they do a price cut it could sell amazingly.
Pro wasn't a half hearted attempt. It was a smart way of going about it so it's cost wasn't crazy. They basically just doubled up on the GPU they were already buying from AMD, with a few slight improvements, and over clocked everything. It's why it's $399 and not $499. It'll also allow them to earn more profit when they do cut the price. In the end, what truly matters, is the Pro is going to easily outsell the X. And when the PS5 comes out, they are both going to look weak in comparison.