Fantasy Grounds – How to repeat a 4d6 dice roll to fall lower

This functionality is not present in Fantasy Grounds as of version 3.3.9.

The only "discard the lowest" functionality that exists in the FG rule set 5e is to launch a d20 with advantage.

However, there are some third-party extensions that add this functionality.

Nor is there a "repeat last entry" command. However, you can create a shortcut in one of the quick slots for any chat console command.

Source: My extensive experience with Fantasy Grounds.