Is your 20 gal a "std" (24" x 12" x 16"), a 20 gal long (30" x 12" x 12") or a 20 gal XT (20" x 10" x 24")?
About the smallest footprint I'd go with for
D. biocellatus would be a 20L. Since this species is benthic/demersal, the larger the footprint, the better. You'll also want to be sure the aquascaping allows for lots of open substrate, but there should be a cave/overhang or two as well, as the fish will definitely use them, esp. at first.
FWIW, fu's do get larger than 4" in captivity, altho they might top out at 5" or so in captivity. WC adult fu's are closer to 6".
Being a bottom dweller, this lionfish generally feeds more heavily on crustaceans as opposed to piscine prey. It's best to start them on live ghosties (gut loaded), then once the fish is eating well for you, you'll want to wean it onto non-living foods (SW fish flesh, shrimp, crab, bivalve meat, squid, etc). Keep the food size about the size of the fish's eye, and feed it 4-6 pieces about 3x a week.
Stay away from goldfish, rosy reds, and other members of the carp family, as they can cause nutritional issues down the road. Likewise, you'll want to limit the use of krill (in any form), as it presents its own set of problems (mandibular malformation/lockjaw as well as CNS disorders).
I strongly recommend the use of a "stealth stick" (see weaning article).
I'd give these a read:
Lions, and Turkeys, and Devils, Oh My! Lionfish in the Home Aquarium
Tools of the Trade: Equipment & Techniques to Convert your Fish onto Frozen