Version 3 will be supported until the end of time. Version 4 was an evolutionary dead end that never took off or saw much adoption.
I'm not sure what value of "much adoption" you're using, but I think it's had great adoption.
All the operating systems I names have fully implemented and mature NFSv4 stacks. Definitely not a MVP to tick a box.
All my file access is over NFSv4 (in a mix of 10 GbE and 100 GbE).
Having GP’s crate expand to support NFSv4 would be welcome but a nontrivial bit of work.