I live on the Island, and I don't think pH would be a problem. The malts you used should be fine with my water (pH wise), but I always add some Gypsum and or CaCl2 to increase the calcium and sulfate levels, and balance the sulfates with the chloride.
In your description of the process however, there appear to be a few steps missing.
You didn't mention a boil. I assume you did boil, and took the gravity reading after the boil.
Did you top off with water after the boil? If so, your gravity reading could be the gravity of the top off water rather than the wort as a whole unless it was really well mixed.
When you added the sparge water, I didn't see any mention of stirring. Except when fly sparging, you should always stir the water in well, as it is the stirring that extracts the sugars.
You also didn't mention draining off the initial runnings before adding the sparge water. This will help with efficiency, as a lot of sugars will be removed from the grain in those initial runnings, so the sparge water won't have to dissolve as much.
Hope this helps.
-a.
You added 8g water (4.5 + 3.5). That should get you about 6.3g.
I assume you boiled, but did you check the gravity before or after the boil? If before the boil