Does DJI make one that charges all at once, or is it just one at a time?
DJI only makes battery chargers that charge one battery at a time. If using one of their battery charging hubs, you'll be able to connect 4 batteries at a time. That makes charging more convenient since you won't have to swap each battery as they finish charging.
The
standard battery charging hub (also available direct from DJI here) can charge 4 batteries in
5 hours:
The
advanced battery charging hub (also available direct from DJI here) can charge 4 batteries in
4 hours. If you use the
Phantom 4 battery charger along with this hub, you'll be able to charge 4 batteries in
2.5 hours.
DJI will not cover damages caused by 3rd party battery chargers. If that's a concern, then this would be your best option if you want to use OEM DJI accessories.
What is the best system for charging?
If the best means the fastest, then I think the
Yx battery charger is the best way to go. It can charge 3 batteries in
1 hour.
While it is a 3rd party battery charger, it has good reviews and I'm not aware of any issues where batteries were damaged. Also, Yx has been making battery chargers for DJI batteries for a few years. Their battery chargers have had consistent reviews over those years. Check out the reviews
here on Amazon to see what people are saying about this battery charger.