Self-similarity on 4d cubic lattice

A phenomenon of “algebraic self-similarity” on 3d cubic lattice, providing what can be called an algebraic analogue of Kadanoff–Wilson theory, is shown to possess a 4d version as well. Namely, if there is a $4\times 4$ matrix $A$ whose entries are indeterminates over the field ${\mathbb{F}}_2$, then the $2\times 2\times 2\times 2$ block made of sixteen copies of $A$ reveals the existence of four direct “block spin” summands corresponding to the same matrix $A$. Moreover, these summands can be written out in quite an elegant way. Somewhat strikingly, if the entries of $A$ are just zeros and ones – elements of ${\mathbb{F}}_2$ – then there are examples where two more “block spins” split out, and this time with different $A$’s.