How can I gain weight with a really high metabolism?

I already have all the muscle I want from cheering, but thank you!

No such thing as “hardgainers”. You’re more than likely not eating as many calories as you think. Try logging a normal day of your diet to see where you are. Scientifically, if you eat 1 to infinite amount of calories over your maintenance then you WILL gain weight. Unless of course you are a special class of genetic humans. Also, note that with steroids/prohormones/medication you need to eat more than you normally would while being “natural”.

I eat 4000 calories/day at 5’3 and 143lb. and I’m gaining a pound a week. My maintenance is at 3000 because of my physically demanding job.

The main thing you need to realize is that in order to build muscle, you have to be at a net positive calorie balance. This means that you have to consume more calories than you burn off all day long. If you have a very fast metabolism, this means you’re burning off calories so quickly that it makes it difficult to keep up with your food intake. So to outsmart this issue so you can start building muscle, you need to look for ways to increase your calorie consumption while decreasing your calorie output.