Let x, y, n all represent positive integers in x^4+nY^4. It seem there is a lot of primes in this set. In fact, even allowing x=1, n=1, we look at 1+Y^4, we see pairs, y=1, f(y)=2, (2,17), (4,257), (6,1297), (16,65537), (20,160001) Possibly an infinite set?
Take the case of x=1, n=2, giving...