My best guess is, that those Clan XP for quests will be awarded after reaching the milestone (30 quest) AND at the end of a period. That way they can prevent a Clan getting to level 5 at mid terms and get more quests and so on...

Ok, but how have some clans already hit Rank 4?

The original clan achievements also count towards experience it seems (i.e. all the ones on the starting clan page that count up to level 20)

Edit: So essentially any established clan that already had most of their clan achievements done started with more experience than the newer clans that were still quite low level

How do we get individual clan xp?

Sorry if this is a necro post, or if it was answered elsewhere already.

By doing clan quests. 

If you have a clan like mine with all end game players its not hard to reach them xp milestone we are on level 17 already )

Can I view my own personal Clan XP? I'm about to leave my clan to find one that participates more often but it says my personal XP counter will teturn to zero.

Yup - in the Clan Member list, it shows it to the left of your activity.

But it's just a cosmetic thing. It doesn't enable anything.