How does one distinguish between swelling/edema and not just fat that has not been removed? (Photo)

Yes, it is too soon to tell what your final results will be. You have swelling, which can last 3-6 months after your procedure. 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

At 6 weeks you still have a fair amount of swelling. It will take up to 6 months before you see your final result. You may improve the swelling with compression garments, massage, and radiofrequency treatments. Later, your surgeon can check to see if any residual fat is left.

Since you are only 6 weeks post op, you will still have swelling.  4.5 liters is a large volume, so there should be some change with that amount.  I know it can be frustrating and it's something you should talk to your surgeon about.  One thing I do with all my patients is take good before pictures and then take after pictures during the recovery.  It's great because you can see changes, even when the swelling is there.  It's important that the pictures are consistent and taken with the same camera, same garments/underwear, same lighting, and same positions.  Taking different views, like side view, 3/4 view, and front view are important as well.
As for going a second round, remember, liposuction is only fat removal.  You have to be sure that the shape you have is due to fat the is directly under the skin.  If you have visceral fat, which is fat around your organs, liposuction cannot reduce this fat.  Also, if your fascia (which is the tissue that surrounds your stomach muscles and makes up your abdominal wall) is stretched out, like after having kids, liposuction will not help with that either.  Liposuction is not always the answer, even if you remove every last bit of fat.
I know you said you had many consults, so I hope these things were explained to you.  You said you opted out of a tummy tuck so you weren't expecting a washboard stomach, but a tummy tuck won't give you that.  A tummy tuck will address other factors that make the belly look big such as loose skin and/or a stretched out abdominal wall.  It will give a belly a better shape, making it flatter, that's it.  Again, talk to your surgeon who did the liposuction.  He or she can best explain what you should expect over the next several weeks of healing.  If you are still unhappy, then get an opinion from a board certified plastic surgeon who specializes in body contouring.  Best of luck to you.  Dr. Jaime Bastidas