Stable vectors in dual Vinberg representations of $F_4$

This paper gives a classification of stable vectors in dual Vinberg representations coming from a graded Lie algebra of type $F_4$ in a way that is independent of the field of definition. Relating these gradings to Moy-Prasad filtrations, we obtain the input for Reeder-Yu's construction of epipelagic supercuspidal representations. As a corollary, this construction gives new supercuspidal representations of $F_4(\mathbb{Q}_p)$ when $p$ is small.