I think these generational distinctions are worthy of further discussion, and there are some gray areas. Geometry is OK, maybe there may be some problems in new gen propulsion systems, but even in the structural part, not to mention the tons of parameters marketed as the innovation of the fifth generation, such as electronics, combat, weapons and ammunition, machine-human communication, decision support systems and autonomy, sensor fusion, etc., I feel that what cannot be bring in 4th generation jets, is not discussed enough in defense forums among international defense enthusiasts.
I never underestimate the fifth generation aircraft, the question I am asking is, what avionics, systems, sensors that you can put in the fifth generation jet but you cannot put in the 4th generation jet? This begs the question, does the force planning of an air force have to consist entirely of fifth generation jets? Is it a necessity to bear these costs or will hybrid force planning continue? Moreover, while groundbreaking new systems in the UCAV field are slowly coming to light.
What the future holds is another matter, but for today and for the next 20 years, full stealth operational capability is already theoretically controversial; in a real medium/high scale war scenario, not all units of the operation are composed of stealth aircraft. Also, communication of these stealth fighter jets with other elements is still a headache. The US has spent and continues to spend billions of dollars on this issue. Geometry is important, radar, thermal and chemical footprints are important, but just as important is the electronic footprint. How much of this will be unique to the fifth generation and will not be included in the fourth generation, and how much of it will be able to conduct joint operations with the fourth generation while maintaining stealth without compromising these features, are questions I have always wondered the answer to.