Decidability of tabularity in extensions of K4

Determine whether tabularity is decidable in the lattice of extensions of the modal logic K4.

Background

The paper situates its results within known decidability questions for lattices of modal logics. Although several properties become decidable in the lattice of extensions of S4, the status of tabularity for extensions of K4 is stated as unresolved in the cited literature.

References

As far as we know, the decidability of tabularity in ${#1{K4}$ remains open .

— Most properties are undecidable even in $\mathop{\mathsf{NExt}} \mathsf{Grz}_t$  (2608.30816 - Chen et al., 31 Aug 2026) in Section 1, Introduction