Size class specifically for portrait 3.5 inch (iPhone 4S) Xcode 6?

An approach that just worked for me was to use the same constraints for all compact size classes but to use a combination of a greater than or equal to constraint and priorities to modify how the views were positioned on the iPhone 4's smaller screen.

I've got a constraint between the top of a numeric keypad view and its superview that is set to be greater than or equal to 160 (with a priority of 1000) and a constraint between the bottom of the keypad view and the bottom of the superview that is set to a constant of 30 (but with a lower priority of 750).

This means that on the iPhone 4 where there's not enough room for 160+ points above the keypad and 30 points below then it's the space below that goes.

Whilst this approach may not work in all cases, I'd encourage you to think about whether there's a set of priorities that will allow your views to adjust in the way you want on the smaller screen.

Swift 3 version of Pavel Alexeev's solution. In Swift you can't use stored properties in extensions, so we apply it directly to the constant property.

extension NSLayoutConstraint
{
    //We use a simple inspectable to allow us to set a value for iphone 4.
    @IBInspectable var iPhone4_Constant: CGFloat
        {
        set{
            //Only apply value to iphone 4 devices.
            if (UIScreen.mainScreen().bounds.size.height < 500)
            {
                self.constant = newValue;
            }
        }
        get
        {
            return self.constant;
        }
    }
}

There is no size class for iPhone 3.5 inch.

So I've made a class category for NSLayoutConstraint to edit it in Interface Builder which is very easy to use:

@interface NSLayoutConstraint (Extensions)

@property (nonatomic) IBInspectable CGFloat iPhone3_5_Constant;

@end

–

@implementation NSLayoutConstraint (Extensions)

- (CGFloat)iPhone3_5_Constant
{
    return self.constant;
}

- (void)setIPhone3_5_Constant:(CGFloat)iPhone3_5_Constant
{
    if ([UIScreen mainScreen].bounds.size.height < 500) {
        self.constant = iPhone3_5_Constant;
    }
}

@end