2008 - top 4 all made the Prelim FinalsTrue but i wouldn't think it's all that uncommon...and we are perfectly set up for it.
2007 - Collingwood made it from 6th (sending West Coast out in straight sets)
2006 - top 4 all made the PF
2005 - top 4 all made the PF
2004 - top 4 all made the PF
2003 - top 4 all made the PF
2002 - top 4 all made the PF
2001 - Hawthorn made it from 6th (sending Port out in straight sets)
2000 - top 4 all made the PF
So.. it's been done twice in 9 years, both times by the team which finished 6th. I would describe that as being "uncommon".