Ok I know a clan has a max number mebers based on level of it. But it doesnt specify if theres aminimum number. Like is a clan leader and one other person still a valid clan (mechanics wise, not ability to get anything accomplished?)
For the lvl 4 quest i THINK you need 3 members besides the leader to do the quest. However i don't know if that is a hard coded requirement. Lvl 3 at least though i know you can do with 1 person in clan.
To do level 4 quest, the clan leader has to be poisoned and kept alive (he/she can't move while poisoned) while a few others kill mobs for quest items. So one person is not enough.