HS4Air was designed to deliver both tangible and intangible benefits by creating a high-speed rail link connecting Heathrow and Gatwick airports to HS1 and HS2. However, no formal, detailed, monetised benefit–cost appraisal exists in the public domain, as the project was rejected by the UK Department for Transport in December 2018 before reaching that stage of development.

Tangible benefits would likely include significant travel-time savings—the proposal identified a 15-minute shuttle between Heathrow and Gatwick, reduced journey times from Manchester to Heathrow (estimated at 1 hour 10 minutes versus over 3 hours by current routes), and improved regional connectivity to Continental Europe. The scheme would also release capacity on London's existing rail network by removing the need for passengers and freight to transit through central London, and could enable direct regional services from the Midlands and North to Europe. Design features such as reusing approximately 40% of the route within existing transport corridors (like the M25) were intended to reduce land acquisition complexity and environmental impact.

Intangible benefits would likely include agglomeration effects—improved connectivity enhancing labour productivity and helping close the UK's productivity gap—as well as strategic resilience through network redundancy and potential national prestige from integration with the European high-speed rail network. Environmental benefits from modal shift away from short-haul flights and road traffic to electrified rail were also proposed, though net carbon impact would depend on construction emissions, traffic diversion rates, and electricity carbon intensity.

Housing and development benefits were identified as a potential offset to construction costs, with proposals for tens of thousands of new homes along the M3 and Medway corridors. However, any credible quantified benefit estimate would require a formal transport appraisal including demand forecasts, journey-time modelling, values of time, and sensitivity analysis—work that was never completed for HS4Air.