To actually gain 4 pounds, you would have to have eaten 14,000 extra calories on top of what your basal metabolic rate, which would be at least 1,200. Unless you did that, don't worry about it. You have the weight of the food, plus added water retention from the extra salt and carbs that usually come from overeating.
If you overeat, DO NOT get on the scale the next day or two. It is not an accurate reflection of what is really going on and just plays games with your head. Just resume your normal routine and weigh in about 4 or 5 days. Frankly, I hardly ever get on the scale. Success is about so much more than that number.
And if your back already is sore, don't double up on the exercise. You just risk not being able to work out for a longer time. And you can't out-exercise poor diet choices. Just figure out what happened and move on. You'll get there.