Water – My RO water filter system has three filters. What is the fourth filter

First, look up your municipality's water department site and see what they specify for treatment. Your sediment filter will work fine, if you replace it when it becomes full. For the chlorine smell, your water likely contains chlorine or chloramines. A loose filter filled with granulated activated carbon (GAC) should remove these chemicals. The water will interact well with the pores of the GAC, assuming you size the filter for the maximum flow rate you expect.

It may be reasonable to filter sediment at the house level, but filter out the disinfectants at the points of use. There's no benefit to treating your non-drinking water, and you'll replace the filters more often if you treat the whole house for chemical/odor/smells.

Humus, or tannin, describes a quite large range of compounds. Unfortunately, this means it can be difficult to treat - any given treatment method may or may not work well for your particular water.

On the cheap end, activated carbon filters can sometimes be sufficiently effective at removing tannins.

The most effective methods are reverse osmosis and anion exchange resins (similar to water softeners), although these methods are also fairly expensive and high maintenance.

For your concerns about microorganisms, the most practical, effective treatment is chlorination. Chlorination can also break down tannins, if given sufficient time and adequate concentrations. After treating the water, the remaining chlorine can be removed with an activated charcoal filter.