For 4 years I have had a large, swollen lump on my arm, inside my elbow, from an IV during dental surgery 4 years ago. I've shown it to my doctor twice in 4 years, and he always says "it's nothing to worry about" but days like today it's very sore and makes my whole arm ache. I lost my job a few months ago and won't have insurance until October. What is this lump? Is it really "nothing to worry about"?

Your doctor ( assume doctor not dentist) looked at it twice, did they do any scams or anything ?

While not normal, if a medical professional examined it, people on the internet with no exam will have no idea what so ever.

At this point, wait till you get insurance and go back to the doctor and be more forceful to have it examined