Here's what Dan Kadar has to say about the selection:
Some though Iowa defensive lineman Christian Ballard was going to be picked in the first round of the 2011 NFL Draft.
For the Minnesota Vikings to get him in the fourth round defines a steal. The Vikings needed a defensive tackle to replace either Pat or Kevin Williams. While Ballard isn't quite as big as either, he projects similarly to Kevin Williams.
They have a full scouting report on Ballard right here as well.
The more I read about the Ballard selection, the more I like it, to be honest. It sounds like he can play either inside or outside (even though the Vikings have told him he'll primarily be an inside player), and sounds like he's the kind of guy that Vikings' defensive line coach Karl Dunbar has a tendency to get the best out of. Getting a borderline first round pick in the fourth round is an outstanding get for the Vikings.