5 horse, you'll notice the difference. You're going to gain low end torque as well, and I'm not familiar with M4, but my Hindle was so much lighter than the stock exhaust. The only problems with full exhausts for bikes, you might gain as much as 10 hp (to the wheel) but you might even end up losing power, I've met a few guys who dynoed afterwards and were dead even or lost 1-2 whp. I think the sound alone is worth every penny, so a slip on might be a consideration....ever pass a bunch of stopped cars behind someone turning left and some joker pulls out in front of you because he didn't see/hear you? With the new can or better yet full race exhaust, pull in that clutch for a sec and jab the throttle. You get noticed and that's a great thing on a bike, much safer.