I have 11 fishes in a 120 gallons tank with 40 gallons breeder as a sump. I usually feed about 2 frozen cubes and a small seaweed a day. I run BB and a decent size skimmer, I rarely ever have nitrate problem. Phosphate is one problem that I always struggle with. I usually use about 2 cups of BRS's regular gfo a month. For your 4 small fishes, I don't think you have to worry about nitrate and phosphate that much. A little GFO would take care of your phosphate problem. Water change and a skimmer would be more than enough to take care of your nitrate.