If it has removable back straps then it’s a gen 4. The gen 4 also comes with 3 magazines where as the gen 3 comes with 2. The gen 4 has a two stage recoil spring while the gen 3 is a single recoil spring. The gen 4 has a reversible mag release while the gen 3 doesn’t.

Teofilo, the Gen 4 and Gen 5 Glocks will have larger "Magazine Release" buttons compared to the smaller magazine release of the Gen 3 and earlier Glocks. Examples are here (notice the Gen 5 has a beveled nose compared to the Gen 4) : google.com/search?biw=1920&bih=938&tbm=isch&sa=1&ei=U2T8WuPaHKHb5gLypbLIBA&q=glock+32+gen+4+vs+gen+3&oq=glock+32+gen+4+vs+gen+3&gs_l=img.3..0i24k1.2392.4959.0.5478.9.2.0.7.7.0.81.154.2.2.0....0...1c.1.64.img..0.9.172...0j0i30k1.0.MKJ4oOWl80M#imgrc=vhcK8lQBKcLIJM:

contact sig with the S/N Phone: 603-610-3000