I was a 10 lb baby and was an easier delivery than my 8lbs 2oz brother. But, yes ultrasounds are estimates and can be a lb off in either direction.
OK phew!! Just worried about giving birth to a 10 pound baby lol!
Ultra sounds can be off. But from what the average is your baby should be 4.5 lbs so you're like a lb off but the ultrasound could be off. If you doctor isn't concerned you shouldn't be.