Iron in water comes in 2 forms. Actual iron oxide and bacterial iron. Both are (for the most part, as you see from the ppm of 4) removed by any quality RO filter. I have seen way HIGH iron/tannin based ppms in the 700+ range reduced to 0-1ppm by good RO units.
The result of these high counts results in you changing the actual RO membrane about every 6 months or a bit more for light use.
The use of Boron or Green sand will remove bacterial iron, with out adding "salts" from water softeners. These units are available from water softener companies, and are simple to instal if you "need" them after a good water quality test. Now then......Do you need one? Not really, if you don't mind changing the RO membrane more often. Boron filters are costly and a recharge of the Boron (aprox every 3 years) is around $300 a pop. Green sand filters are cheaper to recharge, But are costly to start with. Just like the Boron filters. The cost of the RO membranes will be a little over $300 every 3 years without the Iron prefilters and you will still WANT to use the RO after removing the iron.
The positive results from USING an RO system. FAR outweigh the benefits of anything IN the water before you filter it.
With RO, you KNOW exactly what you have in your water!
With Tap and well water. You DON'T know exactly what is in that water.
The cost of a COMPLETE and DETAILED water testing is not cheap and you will most likely choose to use the RO anyway.
So then,,,,,Use the RO and change the membrane when you see the ppms begin to rise over 10+ ppm.
I use an in-line ppm monitor to keep me on top of that. I count the readings after 10 min of high pressure run time, as it refills a 100 gallon storage tank.