Northern Arizona athletes both live and train at 6,900 feet elevation. Research indicates that living in altitudes of 6,900 to 8,500 offer an optimal stimulus for red blood cell count increases (Rodríguez, FA; Truijens, MJ; Townsend, NE; Stray-Gundersen, J; et al. (2007). "Performance of runners and swimmers after four weeks of intermittent hypobaric hypoxic exposure plus sea level training". Journal of Applied Physiology. Am Physiological Soc. 103 (5): 1523–1535.). The plethora of professional athletes who make yearly treks to Mammoth Lakes, CA; Park City, UT; and Flagstaff, AZ can provide hard anecdotal evidence.
Descending to Ogden, UT offers NAU a 9.72% increase in available oxygen. When BYU travelled to San Leandro, CA, they experienced a 15.38% gain in available oxygen (credit to Kenneth Baillie, author of the International HAPE database for oxygen levels). Both teams live and train at an elevation far above where they raced. I don't think altitude was a "limiting factor" in either performance. Would NAU have ran faster if they had raced at sea level? Most likely. That does not mean their performance was hampered racing at 4,100 feet.