A four-stage operational transconductance amplifier (OTA) for ultra-low-power applications is introduced in this paper. The proposed circuit inclusive of frequency compensation requires minimal transistor count and passives, overcoming the traditionally difficult compensation of 4-stage OTAs and bringing it back to the simplicity of 3-stage OTAs. At the same time, the proposed circuit achieves high power efficiency, as evidenced by the >3.7X (>11.3X) improvement in the large-signal (small-signal) power efficiency figure of merit FOML (FOMS), compared to prior 4-stage OTAs (sub-1 V multi-stage OTAs). Thanks to the lower sensitivity of the phase margin to the load capacitance, the proposed OTA remains stable under a wide range of loads (double-sided as in any 3-4-stage OTA), achieving a max/min ratio of the load capacitance of >100X.