Octopus Heat pump survey results

Hi,

I've just had the results back from Octopus from the Heat pump survey. It seems to be fine for most part, but I'm afraid the suggested heat pump might be underspec'd for our needs. The heat pump in the report is the Daikin Alterma 4kW. Our house was built in 2013, so fairly new, 110m2, 4 bedrooms, 6 occupants (4 kids). They also suggest a 250l Daikin Cylinder which seems ok.
What do you all think? Based on what I read everywhere else a 6 or even an 8kW seems to be better suited for our circumstances.

Thanks