So I've been trying to solve this problem for a couple of days now. What I've come up with is this:

By way of contradiction, assume that there are positive integers a and b such that $b^4 + b + 1 = a^4$.

Consider the case when $b \geq a$. Then $a^4 - b^4 = b + 1$, but also $a^4 - b^4 \leq 0$. Thus, $b + 1 \leq 0$. Then, $b \leq -1$, a contradiction.

However, I haven't been able to figure out how to prove it with the case of $b < a$.

Thank you.