Look up the Fitzpatrick skin chart. If you’re darker than a type IV, you need to find a Yag laser. If your hair is dark and coarse, you should get very good results. Underarms is one of the best areas to get done with laser since the hair is usually coarse and underarms are not usually tanned. And yes, the shadow is basically gone once you get this done. I only waxed there before because with my light skin and dark hair, there was always a shadow since you could almost see through the skin even right after shaving; never a clean-shaven look. Don’t have that problem anymore. After 2 treatments already, I have only a few hairs and those are so fine that quickly shaving them doesn’t leave any shadows.
With a type IV, you can use either an alexandrite, diode, or a Yag. Settings will have to be set a bit lower on an alex or a diode, but for underarms that should still do a good job. I would go with those first since they are more powerful. just make sure to find a good practitioner who knows what they are doing so that they can set the right settings and avoid burning your skin. there are various types of diodes (like Lightsheer) and alexandrites (like GentleLase, Apogee, etc). Run a search on this and other forums to see if there are any recommended clinics in your area.