Custom Aspect Ratio with AutoLayout

I figured out.

//Given a childView... 
NSLayoutConstraint *constraint =[NSLayoutConstraint
                           constraintWithItem:childView
                           attribute:NSLayoutAttributeWidth
                           relatedBy:NSLayoutRelationEqual
                           toItem:childView
                           attribute:NSLayoutAttributeHeight
                           multiplier:4.0/3.0 //Aspect ratio: 4*height = 3*width
                           constant:0.0f];
            [childView addConstraint:constraint];