My contractor has put down a oil-base polyurethane (4 coats) on my #2 hardwood floor. I notice that it scratches easily. Can a waterbased poly be applied over top? Also, I have heard of a contractor using an epoxy-like finish called "tarakanov". My goal is to get hard floor surface that won't be scratched by grit in my shoes. Do you have suggestions?

Take off your shoes or clean them before walking on the hardwood.
Consider for a moment that in its simplest description, sandpaper is sand glued to paper. You may as well stick that on the bottom of your shoes since that is afterall, what you are creating.
But to answer your question, yes water based finishes can be applied to oil-mod finishes, after it is fully cured (approx. 30 days)

Thanks for your input Dennis! I still want to know if it is possible to apply an epoxy based substance to creat a very hard surface, as I have heard... has anyone heard of tarakanov epoxy? Or is it not adviseable to create such a hard surface on a wood floor that breathes or moves with humidity.

I personally am unfamiliar with that product, so rather than mis-inform you; I defer to my colleagues here to provide that information to you.