The Pueblo rail yard is a great place to get photos. I saw my first confirmed GE Tier IV locomotive there, BNSF 3770. And now, I can add BNSF 3799 to that list. It was sitting in a small consist.
The ET44C4 differs from the ET44AC like the ES44C4 differs from the ES44AC: the center axle on each truck is not a driven axle.
A major spotting tell of the ET44AC/ET44C4 is the huge radiators at the rear and the uniform angle and unbroken line of the grates underneath them (compare to the photos of the rear grates on BNSF 6286, an ES44AC). In fact, this uniformity was what I saw when I saw Union Pacific 2607 in Kansas that made me chase it down for pictures.