The benefit of micro inverters is each panel or with new dual micro innverters every 2 panels is independantly controlled. The output of each panel is optimised constantly to the light conditions. If one panel is effected by shading, its output will go down, but all the other panels will still be working at there optium output. If there is problem with panel or micro inverter only this one will be effected all other panels will continue to work. The output of each panel can be monitored via the internet once user login setup. The micro inverters have warranty of 20 years due to different design.
The benefit of single inverters is lower cost to install for 4kW system, efficiency is few percent higher, if problems occur inverter is easier to replace. The lifespan of a Solar PV system should be 25 years or more, the single inverter lifespan is shorter than this. If there is failure of inverter then there will be no output from system. If there is any shading the output of a string or the whole array will be reduced. The shading could be from trees, TV aerial, telephone line, chimney, etc.