A lot of people are mis-understanding what it really takes to for a drive to be 4K-UHD compatible.
The physical disk drive only has to be BD-XL capable, or in other words be able to read up to 4 layers on the media which results in the ability to read a 100GB BD-Rom. This alone is not expensive at all. You can get a BD-XL capable drive for less than $40 for a PC, whereas a non-BD-XL drive is around $30.
The real cost for a 4K-UHD Blu-Ray player is in the processing and rendering power required for the 4K image. The next cost is for licensing HDCP 2.0, Blu-Ray codec playback, HDMI 2.0 and all the various audio codecs.
If the PS4 Pro has a BD-XL capable drive it will only require a firmware update for it to play.
Since the PS4 Pro already has the processing and rendering power needed for 4K output the only part possibly lacking is the drive being able to read a 4 layer BD-Disc. The cost of adding this is almost nothing compared to licensing that Sony does not already own.