Vaser Lipo 4 weeks ago. Painful sac on right outer thigh & groin not fluid filled. Feels like an open space under skin? (Photos)

This needs an in-person evaluation. I recommend following up with your Doctor. It is normal to have swelling, which can last 3-6 months after your procedure. For liposuction after care, I recommend adding hyperbaric oxygen and lymphatic massage along with your compression to help decrease swelling. To improve your final results, I also recommend adding vanquish for further fat removal, exilis/venus legacy for skin tightening, cellutone/z wave to make the skin smooth, remove scar tissue, and improve the skin’s appearance. See an expert. Best, Dr. Emer

Swelling after #Vaser liposelection is not uncommon. If you were provided a post-procedure #garment, be sure you are wearing it until prescribed otherwise by your doctor. The purpose of the post-procedure #garment is to provide some gentle #support and to reduce swelling. If it feels too tight or causes pain, take it off. Many patients wear the #elastic garment for several weeks after procedures, as directed by their doctor; some choose to wear it longer for several months for comfort. Massage may also help. Your doctor should instruct you how to properly #massage the areas and will hopefully do so at your follow-up visit. If not, it's suggested you inquire about it. However, in this situation it may be best to also be #examined by your surgeon in-person to better evaluate your healing progress. If you ever have concerns about your #healing, or pain that you question to be unusual, it is important to call your plastic surgeon to discuss these further asked to be examined.

This is unusual and you should see your surgeon again to assess.
It may also be a seroma or haematoma which can be dealt with easily.
Don't delay and get it seen to sooner rather than later.