Looking at the specs of both units it appears that the
P4P+ integrated monitor is essentially the main components (screen and electronics) of the 5.5" CrystalSky, but repackaged for the
P4P+ with a different case and set up for 'integrated' use (ports are on the controller, unnecessary items such as external battery supply, second SD card slot, etc., are stripped, etc.) The less expensive packaging and the fact that the display is being provided as a part of another unit (no separate SKU, power supply already there, etc.) allows DJI to provide the display at a lower cost than when purchased standalone, so $300 extra for what is essentially a (projected) $600-ish 5.5" CrystalSky unit seems like a pretty good deal.