A 4 stage RO system is the standard under-sink configuration: two prefilters to protect the membrane, the membrane itself doing the actual separation, and a final polishing filter on the way to the faucet. Understanding which stage does what tells you which cartridge to change when, and where the extra stages on bigger systems are genuinely worth paying for.
Stages one and two: protecting the membrane
The first stage is a sediment cartridge that catches rust, silt and pipe debris. The second is activated carbon, and it is the most important cartridge in the whole system because chlorine destroys the thin film composite membrane behind it. Skip or neglect that carbon stage on chlorinated city water and the membrane degrades quietly, passing more and more dissolved solids while still producing water at what looks like a normal rate. Both prefilters are cheap and both are changed on the maker's schedule, typically far more often than the membrane. They exist to make the expensive part last.
Stage three: the membrane
The membrane is where reverse osmosis actually happens. House pressure pushes water through a semi-permeable film that lets water molecules pass and rejects the great majority of dissolved solids, including sodium, nitrate, fluoride, arsenic and lead. Everything rejected leaves with the reject stream to the drain, which is why an RO unit sends some water to waste. The membrane fills the storage tank slowly and steadily, which is why the system has a tank at all. Membranes last for years when the prefilters are honored, and are replaced when production falls or the treated water's dissolved solids start climbing.
Stage four: the polish
Water sits in a bladder tank between uses, and stored water picks up a faint flat or plasticky taste. The fourth stage is an inline carbon polishing filter between the tank and the faucet that removes it, which is why it is changed at roughly the same time as the prefilters. It is the stage that makes the difference between water that is technically clean and water people enjoy drinking. If your RO water tastes stale, this is almost always the cartridge overdue for replacement rather than a membrane problem.
Four stages versus five, six or more
Extra stages are usually a second carbon prefilter for more contact time, a remineralization cartridge that adds back calcium and magnesium for taste and pH, or a UV lamp on the outlet. None of them change what the membrane does. More stages also mean more cartridges to buy and more connections to leak, so judge a system by the membrane rating, the prefilter quality and the cartridge availability rather than by the stage count on the box. Vendor reverse osmosis ranges list stage counts and membrane production ratings side by side for exactly this comparison, and our RO system installation page covers fitting one.
Questions people ask about 4 stage ro system
Is a 5 stage system better than a 4 stage one?
Not inherently. The extra stage is usually a second carbon prefilter or a remineralization cartridge. The membrane does the separation and its rating matters far more than stage count.
Which cartridge do I change most often?
The sediment and carbon prefilters, on the maker's schedule. The membrane lasts years provided those prefilters are changed, since chlorine reaching the membrane destroys it.
Why does my RO water taste flat?
Reverse osmosis removes the dissolved minerals that give water flavor. An overdue polishing filter makes it worse, and a remineralization stage is the usual fix if it bothers you.
Does a 4 stage RO system remove fluoride and lead?
The membrane rejects the great majority of dissolved ions including fluoride and lead, provided it is intact and the prefilters have kept chlorine off it.