The Ursa 4K PL Mount is fixed, not designed to be user removable, that said, if you do go this route, yiu will need a Micro Torque driver and the correct bit. See the instructions for removing the mount on the Ursa Mini 4.6 PL camera. I do not think the locking ring cst is adjustable.
That said, I would get a caliber and measure the thickneyof the lens PL mount ring. Compare this to the Arri standard measurement:
ARRI PL mount. Thickness of flange face to front of lugs clamping face is 2mm. Outer diameter is 69mm. Diameter to bottom of opposing inner cutouts for the lugs is 63mm. Diameter to bottom of locator pin cutouts is 62mm. Lugs are about 23mm broad at the outer diameter. Diameter of each locator pin cutout is 3.2mm. Fillet radius at corners of lug cutouts appears to be 2.75mm. Outer diameter of rear shoulder is 54mm.
If the lens PL mount does not meet these specifications, inform the seller and ask to return the lens, as it does not meet the required specifications
That said, if it does meet these measurements, have a second check on thI had a PL mount kens thst fit on one PL mount correctly, but would not tighten up on another. Turned out, the lens was not seated correctly, make sure you insert the lens centered on the mount and square to the camera, and thst thenlens inserts into the mount all the way, and the flange slot is fully engaged on the pin. I had an issue with a little debris on the camera mount, that kept the lens from seating.
Good luck