It seems that it works with JRuby, not with MRI.

> We recommend using JRuby 9.X+, with the majority of our testing currently against 9.1. JRuby 1.7.x may work, but has been untested since 4.0.0.pre6.

Your question is still appropriate because they tested with 9,1 and JRuby is at 9.4

About MRI compatibility see https://github.com/shoes/shoes4/issues/1235