WOW, thats a low PH. you got some good feed back here on places to buy once you figure out the type system you need. the 4ppm nitrates is not bad, the RO membrane will easily remove that. since you have well water, you might have low water pressure. normal city water pressure is about 60-70 PSI. RO membranes need at least 60-70 psi to produce filtered water at that 4:1 waste to product ration filterguy mentioned. the lower your house water presure, the more waste and less filtered water you get. thats why some units come with a built in booster pump to bump the pressure up to like 85 psi or more, and get better results.
I just use a basic 3 stage. i micron carbon filter, 1 micron sediment filter and the 50 gpd RO membrane. it has worked fine for me. my water has a high PH from the tap ,so i dont have the same problem you do. filterguy is correct, you could aerate the RO water and then pump it thru a DI if you think you need it that clean. but then you need another pump, powerhead will do, to run thru the DI resin filter, or gravity feed if your storage barrel,(brute trash can) is elevated higher than the final storage barrel.
since you have home well water, you dont have added chlorine or phosphates water companies add to treat their water. the carbon filter will still remove any chemical pollutants from your ground water, and the sediment filter will get the fine particles, then the 75gpd filmtec membrane will remove 98% of the bad disolved nitrates, phosphates, and silicates, along with the good alkalinity calcium , magnesium and carbonate compounds.
thats pretty good. been fine for me. but next i store the filtered water for my daily makeup, in a barrel to which i added a 1/4 cup of ARAGAMITE, fine powdered araganite, to remineralize it with calcium, carbonates, magnesium strontium... trace elements etc. it raises the PH a little and is better for the reef with the added alkalinity. most salt mixes assume you have a certain level of natural alkalinity in the water to start with. or you can add an RO prep re-mineralizer, like kents. it completely disolves. the aragamite disolves some to its saturation level of about 1 or 2 DKH, then the rest sits on the bottom to disolve in the next batch. either will raise the PH of the output water. but DO NOT run the treated remineralized water thru a DI resin filter. a mixing air stone would be good too. although the aragamite might not settle and remain milky looking.
i would stick with a 75 gpd size RO . they are the most efficient. i hear the 100 gpd membrane only removes 92-94% of TDS.
I know others in the club have been very happy with systems from airwaterice, and buckeye field supply, dr. mac, and filterguys look good too. so after deciding what you need, the rest is just shopping.
so consider your TDS and water pressure next.