I mainly use Arducopter, and there are many more knowledgeable pilots on here than myself, but I took a look at those Acecore Technologies drones you mentioned. They use the Cube Orange, one of the most advanced, stable flight controllers on the (civilian) market. I can’t think of anything you would gain by replacing it with a Pixhawk 4. In my view, that would actually be going backward. 
Is there anything specific the Cube Orange can’t do that you need it to do?
You could certainly put a Pixhawk (or any PX4 / Arducopter compatible FC) on a DJI drone in lieu of whatever version DJI controller the drone came with. I’m actually doing that right now, refurbishing an older DJI S900 with a Cube Black setup. But in some ways it might not make sense to purchase a new DJI system, for example, just to rip out the controller and put in a Pixhawk. Unless you really prefer everything else about the DJI setup, then I suppose one upside is, you could always sell the DJI flight controller afterward.