Recently called M4 for a replacement can for my SV. There was slight communication error and the wrong part was shipped. Kyle and the guys at M4 went out of their way to make it right. Both my bikes run M4 exhausts and this situation solidifies why.
I have had noting but great experiences with M4 as well. I did a custom set up on my RC51 and the guy at M4 took the time to get me inlet tube sizes and answer all of my questions. I have had their exhausts on 3 bikes now and they are great.
Just a quick addition to top it all off.... When I installed the system, I didn't torque the o2 sensor plug all the way. Well, it vibrated loose on my test ride today. I called M4 and they put another one in the mail no questions asked. Great company.