Answer
{s, t, u, v} = stu, sut, tus, tsu, uts, ust,
tuv, tvu, utv, uvt, vut, vtu,
suv, svu, usv, uvs, vsu, vus,
vts, vst, tvs, tsv, stv, svt
Work Step by Step
4!/(4-3)! = 4! = 1*2*3*4 = 24 combinations
{s, t, u, v} = stu, sut, tus, tsu, uts, ust,
tuv, tvu, utv, uvt, vut, vtu,
suv, svu, usv, uvs, vsu, vus,
vts, vst, tvs, tsv, stv, svt