How is the triferric tetroxide of iron possible?

If oxygen has a valency of $-2$, $4$ of it would result in $-8$. But iron doesn't have a (variable) valency of anything that goes to $-8$. Then how is $\ce{Fe3O4}$ possible?

In regards to $\ce{Fe3O4}$, according to the Wikipedia page Iron(II,III) oxide,
contains both $\ce{Fe^2+}$ and $\ce{Fe^3+}$ ions
A 2-dimensional diagram of $\ce{Fe3O4}$ is below, from the relevant ChemSpider page: