Full Idea
The system S4 has the 'reflexive' and 'transitive' conditions imposed on its accessibility relation - that is, every world is accessible to itself, and accessibility carries over a series of worlds.
Gist of Idea
The system S4 has the 'reflexive' and 'transitive' conditions on its accessibility relation
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.19