I would really hesitate and I understand the parental hesitation to go the route of surgery for a 4 year old until you've exhausted other options. Sounds like you're on a good track. Are you able to do a sinus wash - neti pot or something? That could be really tough with a 4 yr old but can be very beneficial. how about regularly inhaling a steam/mist with just a tiny bit of tea tree and cinamon and thyme in it?
I just made it thru a winter without taking Antibiotics for a sinus infection for the first time in many years. Besides the sinus wash, I have been taking spirulina. If you search for spirulina and sinus on google you should turn up the info regarding the clinicals. Something about cytokines reducing inflammation there I think. I also take lots of omega-3s. oil pulling and omega-3 supplementation has really helped deflame my sinuses. If your child is like mine and won't eat fish or very many nuts, you could look into the chewable oil capsules. we have nordic naturals strawberry chewables and my very fussy eater will chew those but ultimately spits out the shell.
good luck, truly. that's gotta be tough with a little one, my heart goes out to you - keep us informed. - bd