If US decides to make any new 4.5 gen fighter, it can actually make it within 3to4 years to IOC standard and will very potent and mature battle ready/tested FOC standard 4.75 gen fighter by 2030 itself and won't take 2035 to mature.
Why? They already have very mature 4.5 gen fighter LRUs like GaN Aesa, EW Suite, Turbofan engines, carbon composites, mission computers, control laws, etc...
The world's first 4th generation F16 flew from concept (1972) to first flight (1975) in just 3yrs and was a potent mature platform by 1982. That's some 10yrs only in which many new technologies like RSS frame, Fly by Wire, were developed.
Currently all these technologies are available off the shelf and just need an Air frame which can be developed and tested on advanced computer softwares like CATIA (unlike during development of F16 which had to be tested physically over the years painstakingly) and all above tested and matured LRUs integrated to it. So for US the development cycle is very small for 4.75gen fighter...