Physically, they use the same USB-C cable. Functionally, they’re almost the same. Thunderbolt 4 was released almost as a patch to Thunderbolt 3, ensuring that everything worked properly. If your laptop has Thunderbolt, you should probably be able to buy either a Thunderbolt 3 or Thunderbolt 4 dock without any problems. Since Thunderbolt 3 is being phased out, you’ll probably find more discounts on the older Thunderbolt 3 hardware. (For the gory details, see our roundup of the best Thunderbolt docks.)