4 red bumps on my arm that form a diamond shape, what is this?

Insect bite: Often, insects will cause a clustering of red bumps in the same close vicinity when they bite. The cluster could look like a diamond. The classic example of clustered insect bites is bedbugs. I am not sure what is causing your bumps, but you could be having insect bites. To be certain, you should see your pcp or a dermatologist.