This is where system design comes into play, an experienced PV company will design a system that will work at it's optimum for the customer, it is not just a case of bolt 4KW of panels to a roof then wire it up to a 4KW inverter and away you go.
Of course it will work, but it could work better if designed well for any given situation.
Inverter choice and size is probably the most important factor in getting the best returns from any PV system, (there are of course many other factors too), if the inverter is too large then it will be later starting and shutting down earlier every day, and on cloudy days may not start up at all, where as if it is too small you may miss all the "higher peaks" of production and potentially stressing the inverter shortening it's lifespan.
Even though there are several design tools for inverters available for the professionals, each and every system is different, so experience plays a large part in good system design, there also is the UK weather to try and predict too, even though it is good for PV production it is very varied.
I do agree with the other comments from the experts, that it is normally in the customers interest to undersize an inverter, but as already written by how much depends on many factors.
. The topic is still important today though as i'm sure you'll agree.
