I've never heard of a consumer router that limited bandwidth per port, certainly not by default. That would typically require a far more sophisticated router, and most certainly something you'd have to manually configure and explicitly enable. More likely, the fact that 6 divided by 4 equals 1.5 is merely a coincidence.
What happens if you connect directly to the modem? Do you see a significant and consistent increased in speed? If you do, then obviously it's not a problem w/ the ISP.