I've travelled down a few times of 4th of July and haven't seen too much of a difference as far as crowds. It's low season so the amount of people there is already small. There might be more Americans because it is a holiday in the US. However, it's a short holiday and most people tend to stay close to home for fireworks celebrations. And/or don't go further than STT simply to avoid the customs and immigration hassle for what is probably only a 4 day weekend trip.