Depends on your budget. Pets at home is not the cheapest. Is this for one or two buns? Look on the net, HomeandRoost do some hutch/run combos, 4,5 or 6 ft. (I've not used them so it's not a recommendation , I've just been researching runs myself)
4x4 run is not big enough.
If you're getting a new 2 storey 6x2, why not sit the run in front rather than on top. A ramp hole takes up space.