After your boil, before you pitch yeast, see how much wort you have in the fermenter. if you are low you can boil some water (just to sanitize it) cool the water as you did the wort then fill your fermenter up to just over 5 gallons, in my case i fill to around 5.25.
Then pitch and shake the dickens out of it.
This still doesn't explain why your OG was low. If you had too much water I could see why your OG would be low, but not vice versa.
Did you calculate the offset for your hydrometer to temperature? Mine is around .001-.002 low at 70 degrees as its calibrated at 60. This still doesn't account for the difference however.