If the water is heavily chlorinated but still within the legal 0.5-4.0 PPM of chlorine, can the risk for Legionella be greatly reduced while the water being still safe to drink?
Chlorine is not very effective at controlling Legionella. Here is a review of chlorine from the EPA Technologies for Legionella Control. When asking what PPM of chlorine would be considered adequate to kill Legionella, you need to look carefully at the studies. Lower concentrations can kill a pure culture of Legionella in a jar in a laboratory, however in the environment, especially a plumbing system, much higher concentrations are required to reduce Legionella growth. For a shock disinfection to address high level of positivity or for building commissioning often chlorine concentrations in the 30-50 ppm ranges are used. This can kill Legionella, but re-growth will typically occur within a couple of weeks after the shock. This is because 1) the chlorine will not reach all parts of the plumbing system due to biofilm, sediments, and system construction 2) Legionella can live inside other organisms and studies have found resistance up to 50 ppm of chlorine because of this relationship, and 3) Legionella can re-enter the plumbing system from the public water supply.
Some facilities have tried to operate with an elevated continuous level of chlorine in the 2-4 ppm range (just under the EPA drinking water level). This is typically not successful and leads to early corrosion and failure of plumbing systems because of the high chlorine levels. These chlorine levels can also have other unintended consequences such as elevated disinfection by-products. Here is a 2011 review paper by Lin which is a good reference and you will see there under chlorine that the hospitals which used chlorine had switched to another method (bottom page 4/top of page 5).
So for chlorine, we would not recommend it be used as a continuous disinfectant because there are other more effective and less harmful options. Chlorine can be used as a shock disinfection at high levels, but again understand re-growth will occur quickly after the shock is completed.