What is this quad-jet plane at El Paso Airport?
It's the McDonnell 220, registration N4AZ.


(Source)

Wiki says


  The McDonnell 119/220 was an attempt at making a business jet by McDonnell Aircraft in the late-1950s. It had a configuration that was unique by bizjet standards, with four jet engines mounted in individual pods underneath a low wing; it could accommodate ten passengers in a luxury executive configuration but could carry as many as 26. 
  
  ...
  
  The McDonnell Corporation used the airplane as a VIP transport for a few years before donating it to the Flight Safety Foundation's research facility in Phoenix, Arizona. As of 2020 the single prototype is still extant, registered as N4AZ and stored at El Paso International Airport.


Only one such plane exists. Its first flight was in 1959.

Here are some pictures.