Not all ventilators are designed to function well at high altitude. Piston ventilators don’t have to compensate for altitude as the volume of the piston is constant regardless of the altitude.  It is blower based devices that rely on the control and measurement of tidal volume, which are impacted by changes in gas density.
Most ventilators can be calibrated at the factory to a given altitude – so if you know you are going to use it at 4600m – you can have it calibrated to that specific altitude.
A manufacturer may say ‘our ventilator compensates for altitude up to 10,000 feet.’ What this often means is that because of loss of compressible volume in the circuit – the set VT of 400 is only 360 ml, and when you go to altitude at 10,000 feet the tidal volume increases to 440 (80 ml or 20%) but it is still +/1 10% of the set VT and therefore meets the ASTM standard. That is not true compensation
One additional consideration is oxygen partial pressure delivered. At sea level, assuming normal ventilation (i.e. normal CO2), 50% FiO2 corresponds to an alveolar partial pressure of oxygen of ~300 mmHg; at 15,000 feet (4500m) delivery of 50% FiO2 corresponds to an alveolar partial pressure of oxygen of ~140 mmHg
(Tourtier et al, Trauma and Acute Care Surgery,  2010)(Rodriquez et al, Trauma and Acute Care Surgery, 2009)(Blakeman et al, Trauma and Acute Care Surgery, 2014)