Diadem requires 2 people, but only 1 has to actually be in the FC.

Workshop projects require 4 people, but like diadem, only 1 has to actually be in the FC.

Creating an FC and purchasing a house for the FC from 4.2 will both require 4+ people, but only in the FC member list. They don't need to actually be online at the time and the actual registration purchase is only done by 1 person.

Activating actions/creating new actions via the aetherial wheel only takes 1 person.

Gardening only takes 1 person.

Decorating only takes 1 person.

So basically there are no 4 people FC activities or anything that actually requires 4 FC members to be online at the same time.

The 4 person requirement for workshop is just a major inconvenience with no real point as it doesn't require FCs to work together and I'd really love to see it removed. Going the other way and actually changing it to require 4 FC members would just cause more problems for people and also penalise legitimate small FCs.