4 to 6 years is possible, but more of a best-case scenario.
If you look for example, the nuclear reactor built in France in the 70ies and 80ies took generally between 5 and 9 years to build, with the newer and greater capacity reactors taking significantly more time.
https://en.wikipedia.org/wiki/List_of_commercial_nuclear_rea...
Keep in mind that it was at the pick of Nuclear reactor construction. These are really large projects with tons of suppliers and contractors to coordinate. There is a lot of experience and institutional knowledge needed, but which has been unfortunately partially lost with the significant slowing down of new constructions in the late 80ies/90ies.
Also, this slowing down also had a significant effect on the supply chains, often leaving very few companies key components/sub-systems.
Flamanville 3 is definitely a poorly lead project and a big outlier, but, a country re-starting its nuclear construction, the first units would probably take ~10 years to come online.
