There could be a couple of reasons for the reading changing like that, though 4 points is unlikely. First, your priming solution wasn't mixed well into the beer, and you happened to take a sample that had more sugar. Second, and more likely, your hydrometer was sticking to the side of the test jar. Do you spin the hydrometer? You should be spinning it before you take the reading, and I'd take 3 readings from the same sample to be sure.
As for bottle bombs, you're almost certainly safe, especially with Grolsch bottles. There are a lot of things that will affect your FG. Mash schedule, the yeast you use, the temperature you ferment at, and especially the quantity of yeast you pitch.
What yeast did you use, what was your OG, and what were you expecting the FG to be?