local MatTable = {
    [1] = Material("lalala"),
    [2] = Material("lalala"),
    [3] = Material("lalala"),
    [4] = Material("lalala"),
}

...

function SetTrail(...)
    local Mat = MatTable [ self:GetOwner():GetNWInt("currtrail") ]
    local trail = util.SpriteTrail(self.Entity, 0, Color(0, 255, 0, 255), false, 5, 4, 4, 1/(5+4)*0.5, Mat)
end