Volume is much less important than gravity when it comes to the beer turning out as the recipe intended. When you add top off water after cooldown and prior to fermentation, you ideally should add well mixed water until you hit the recipe's intended OG, NOT to some arbitrary water volume.
Now that you've already fermented, you really shouldn't add water for a few reasons. First off, water straight from the tap has quite a bit of dissolved O2 in solution that will slightly contribute to oxidation. Just the act of getting the water into the beer also presents some slight oxidation issues.
Second off, you no longer know exactly how much water you need to add, since you can no longer take a OG reading. It is 0.5 gallon if the rest of your process was 100% right, but it never is, so you probably need to add slightly less than 0.5 gallons, but no telling exactly how much at this point.
I would just stick with the 4.5 gallon yield at this point. The beer will still be in the balance the recipe intended, it is just going to be a bit maltier, bit more bitter, and a bit higher ABV, but not enough to throw the whole thing out of wack. It should still be a good beer.
Good luck!
