I have never tried to stream Amazon at 4Mbps personally, but earlier this year my PTCL connection was going through some issues and I was able to stream Netflix flawlessly at 4Mbps just fine. And from my personal experience of usage, Amazon gives faster streaming, when browser is considered at least, as compared to Netflix on my PTCL connection. As in States, they all are same to me. So, going by that, I'd assume if Netflix was able to stream at 4Mbps smoothly for me, Amazon should be too. It goes 1080p within a minute on a browser, streaming experience on devices like TV is even more smoother and goes on 1080p much faster because no other programs allocating the bandwidth and what not.