What Glennjanie was saying about having only three fixtures on a water supply pipe is just what your local plumbing inspector considers "good practice", and it might even be part of the plumbing code as well. If you have more than that, the concern is that simultaneous use of all four fixtures at the same time will result in none of the fixtures having sufficient water pressure. It's not like there's any underlying scientific reason why you shouldn't have more than three. After all, if you look at the men's washrooms in schools and office buildings, it's common to see more than four urinals all supplied by the same 1/2 inch water supply pipe. They're counting on the fact that you're never going to have all the urinals flushing at the same time.
So, if you keep that in mind, and try not to use your attic sink when you can hear water being used down stairs and vice versa, then you should be fine.
Water has a density of 62.4 pounds per cubic foot at sea level. The surface area of the bottom of a cubic foot is 144 square inches. Therefore stagnant water has a hydrostatic pressure gradient of 62.4/144 = 0.4333333 psi per foot of height.
So, the water pressure in our upstairs sink will be about 8 feet time 0.433333 psi per foot or 3.47 psi less than in your downstairs sink. Winnipeg's water pressure is typically about 60 psig as measured at the water supply for the city, so a 3 1/2 psi drop is nuttin. In cities like San Fransisco, where you have 200 foot high hills, they have to have much higher water pressures because the people living at the tops of some of the hills there wouldn't get any water at all otherwise.