Answer
True
Work Step by Step
Recall: A function $y=f(x)$ is decreasing if $y'<0$.
Given: a differential equation $y'=-1-y^4$.
Since $-1<0$ and $-y^4\leq 0$ for all $y$, it must be $y'<0$.
Thus, all solutions to the differential equation are decreasing functions (TRUE).