I always read about guys eating crazy amounts of calories like 4000 a day and I wondered why this is neccessary?
One needs to be in a caloric surplus to gain muscle. A variety of factors dictate necessary maintenance (and surplus) calories but the bottom line is that one needs to be in a surplus of calories, not just protein, to gain muscle.
I suppose an apt, simple analogy building a house. The bricks are the protein. You have a bunch of bricks lying around but you need workers to put the bricks together. Those are the calories.
An aside: You provide a stimulus with the training to build muscle. Your body is going to adapt to that stimulus -- it can build muscle to be able to deal with the next bout of stimulus or it can just break down. Building muscle isn't really a priority of your body. It would rather not build more muscle than it thinks it needs -- it's a pretty high-maintenance tissue. As an athlete/bodybuilder, you want more muscle than your body wants you to have.
Providing sufficient (i.e. above surplus) calories gives your body the energy it needs to build muscle. Because you are putting it an a 'happy' environment with all the food coming in, the body will take the route of building muscle since it has the means to do so.
Without sufficient overall calories, the body will not be in an environment to build muscle.
my calorie intake is kept at around 1500 a day.
For an adult male, 1500 calories is almost invariably insufficient total calories to bulk on.
Do I need to add more calories to get bigger or am I ok because I'm eating enough protein?
Eat more food. A large part of the industry is pretty hinged on building muscle without losing fat. If you can figure out the way to do this perfectly, you'll be rich.
With that said, find something that works for you. Many people like Martin Berkhan's protocal from leangains.com. You can carb cycle -- there's plenty of options.
What's consistent between all of those options is that you need to be in a caloric surplus. The key is figuring out a protocol that fits your lifestyle, habits, etc. Anything that isn't retarded will work, find one that works for you.