the reducers suck. See the issue is that the O.D. of the 4-4 pipes is only 1.5 inches or so. Most slip-ons are close to 2 inches. What I did was get small diameter exhaust pipe and open one end to fit the slip-on and the other to either go in or over the header pipe. From there, the seams need to be welded. I tried to expand my header pipes but one cracked a bit and that was that, I stopped that trick!
It is not pretty but if you polish the adaptors well with something like nev'r'dull, it can get really shiny and it much harder to notice.