Yes and no. Installing last version of android on a pixel 4 is most likely absolutely fine. And keeping at least security support is likely not a big deal. 3 years of security update support it is clearly a finance department decision. Why 3, why not 3 and half? Why not 4?

Just because they need predictability in sales, and they attached the support to the “classical” number of years after which you’d like a customer to buy a new tech product. 3 years has always been a magic number for hardware companies, since forever