A note on Fermat's Last Theorem for $n=4$

Abstract

Fermat's Last theorem (FLT) famously states that the equation $x^n+y^n=z^n$ has no solution in positive integers $x, y, z$ for any integer exponent $n>2$. But does this theorem have a quantitative version? Upon initial investigation we discovered an infinite sequence of integers $(x_n, y_n, z_n)$ with $x_n^4+y_n^4-8=z_n^2$.