The 4006 is just a slight variation on the 5906, basically a caliber conversion of a gun designed for 9x19 to allow it to shoot .40S&W. And the G22 beat it to market, which was probably the opening salvo of an Era of Very Bad Feelings between the two companies that culminated in S&W ripping off Glock's gun, lawsuits, and a multi-year stretch where you couldn't order S&W and Glock guns from the same wholesalers. I can't remember which one Ruger was on the outs with, back then...I think Smith?...so basically you'd be like "Well, I can order Rugers and Glocks from XYZ wholesale, but to get 642's and 5906's, I have to go to ABC wholesale..." Good times, good times.