Normally I don't Google for people but here's one I found almost right away
Amazon.com: Venom DJI Phantom 4 Venom Pro Charger for Drone: Toys & Games , and there are more.
Another easy option for someone who wants maximum charge rate and has the typical 3 batteries is to just buy another DJI AC charger to add to the one you have now, which will allow you to charge two batteries at once while the third is flying your drone. That's one advantage of an inverter, lots of options.
Sorry, but I did search for better ones a couple months ago with no joy, but that's a nice 4X charger you found. However to use it mobile you'd need a 500W inverter, and like AyeYo mentioned, you'd need to be diligent about monitoring the car battery level, pulling 30A @12VDC from the battery for an hour, and don't forget the hassle of connecting direct to the battery
The other option you mentioned is something I have been considering. When I buy a
P4P this Spring I would have two of the P4 chargers by default, since I don't plan to sell my P4, keeping it for backup. Given the size and power draw, I think the 4X charger would only be appropriate for home use. Unless something changed, I wouldn't use it in the field often enough to justify storing such a large device in my trunk. Since I have 5 of the P4/
P4P batteries, a second charger may suffice with a 300w inverter for the car, not to mention the DC 90W charger I have. However, car charging
usually always requires the motor or ignition accessory to be on to enable power to the cigarette lighter. Portable generators are nice, but that's not in the cards for me.