It's not really arbitrary though, four is the minimum number of people you need for fc stuff. Which likely based on the minimum number of people you need to field a complete light party. (WoW's is similar, except you need 5 people)
Removing it primarily, and pretty heavily, benefits the people who have shell FCs to buy extra houses with.

For others, keeping it there has the benefit (to SE) of encouraging people to grow their FCs so that getting four people is a non-issue. Either by building a group in core hours, getting heaps of members, or even merging smaller FCs.

Making it more interactive could make it more fun, but probably worth keeping in mind that it should influence the outcome which could be bad.