Sony's goal about the backward compatibility of Playstation 5 is that the previous software works optimally on the new hardware, but at the same time the company is studying new ways through which it could work better.
Sony has previously revealed that backward compatibility is a key feature in PS5, although we still don't know exactly how it will work. Since there is still no clear information, some have looked at the company's patents to try to find out more. And in fact, Sony has recently updated a patent which illustrates how it intends to make PS4 games work on PS5.
As noted by Twisted Voxel, the patents page shares some of the main methods that Sony will rely on to ensure that the games of the now next old gen have no problems with the new hardware. Although the description of the patent is full of technical jargons, there are some elements that are worth discussing.
First of all, Sony will play the game in question on the old Playstation 4 hardware in order to detect its performance characteristics. Next, it will run it on the new hardware to verify a point of convergence between the data of the two versions.
The objective of this verification is to synchronize the operational parameters of the new device to achieve the same performance levels seen in the previous hardware.
Although this may seem counterintuitive, given the power difference, Sony probably wants to make sure that no new problems are created. Although modern consoles are increasingly similar to PCs, each generation still relies on proprietary technologies that can differentiate platforms.
Furthermore, this would only be the first phase of the procedure. Once established the stability of the old gen game on the new hardware, Sony will begin to optimize performance to see if the game could benefit from the greater power of the new hardware. This is done until the application crashes, to determine its ceiling beyond which it cannot go.
It is a process in essence comparable to PC overclocking, whose optimization and tuning are a fundamental part. The description does not mention any changes to the original code of the old Playstation 4 games.
Whether or not the process in question will involve all current-gen titles is obviously still to be seen.