How 2 get constraint to fix pos twix 2 widgets

Using XCode: Version 8.2.1 (8C1002)

I have a view that contains several UIImage views and several TextFields. One UIImageView sits in the lower left part of the view near the alignment guides. Above that UIImageView sits a TextField whose bottom edge should be 4 pixels above the UIImageView's top edge. So I set a vertical constraint as:

Constant <= 4

Priority = 750

Multiplier = 1

//--------------------------------------------
//| enclosing_view
//|
//
//
//
//
//
//  ------------
//  |     00    |
//  |___________|
//    <constraint is here>
//  -------------
//  |            |
//  | image_view |
//  |____________|
//
//|_____________________________________________

This is in the smallest device view (iPhone 4s). As I go up the device views the TextField jumps to near the center of the view. The constraint has jummped to over 70 pixels! This is the only constraint on the entire ViewController. That does mean I'm missing some constraints but this is the ONLY constraint! What could be overridding the <= 4?

So two questions:

1) Why isn't the constraint obeyed?

2) How do I get a layout to obey a constraint to force two widget to track each other?

Any help appreciated.

TIA

Clark