I think polymeric is more for paver joints. I don't know how it would help to keep the paver flat. I particularly wanted resin pavers as opposed to concrete as I didn't want to make the ground flat beneath them and some of mine are curved. This isn't an issue for me as they are covering a disused flower bed, are held in place by a concrete sidewalk and I have plant pots on them. Heavy concrete pavers might work better for your needs
I made sure the ground was wet, threw them down on top of the grass, use round up as needed. I'm sure you can scrape the grass out, level it, but what I wanted it for did not require the extra steps.
I would not recommend them for that use and I certainly would not put a fire pit on plastic pavers. We use ours along the side of the house where we keep our trash cans.
The hold up pretty well. I bought mine last year and put them down immediately. I only put heavy black poly under them. They are kinda honeycombed underneath. I'm thinking with the poly and sand they would sink down to the poly anyway. They blow up when you get after them with a leaf blower, but you just push them back into place. I put concrete 8" x 16" paving stones around them. I'm thinking of drilling holes in the honeycomb and putting some quick-crete in the corners to make them heavier. Carol
I USED THEM FOR TWO HEAVY (AT TIMES) TRASH CANS AND THEY WORK FINE. PAVERS SHOULD WORK WELL FOR A PATIO OVER A GOOD, LEVEL SAND BASE.
You 4 by 5 area is 20 square feet. One package of pavers cover 24 square feet.
I don't know about ground freezing but as flimsy as they are in regular winter weather I'd say they would be awful. Not sturdy at all.
fire & heat are not plastics best friend. They will melt.