© 2016 SPIE. This project involves the design, analysis and verification of the optical performance of the 4mm receiver channel for the 20 m telescope at Onsala Space Observatory, Onsala, Sweden. The 4 mm (75 GHz) receiver is a newly proposed channel designed to be installed in parallel to the existing 3 mm (100 GHz) channel targeting new science targeting longer wavelength spectral line analysis of the ISM. Gaussian beam mode analysis is used to produce the fundamental optical design of the system. The design is then analysed more accurately using physical optics. We report on the comparison of simulation and measurement and verification of the system design.