As a professional investor, my analysis says that 4kW for £7400 is "worth", to me, RPI+4% per year.
So about 7-8% if inflation remains around the Bank's target of 3-4%. Proportionately more if inflation turns out to be higher.
But bickering over the level of returns put to one side: solar is still a good investment which, on a risk/return-adjusted basis, beats just about anything else available at the moment (as I stated yesterday on another topic).
A relative of mine has recently received a similar quote for £7-something for 4kWp, and their illustration claims about 12% return. I regard it as more like RPI+3%; about 6-7% annually if inflation averages 3-4%.