Ben Q and Sunpower do 327 watt ones, which would give you a 3.92 system, which is as near to a 4 kwp system as makes no difference.
However, you need to pay a premium for these more powerful panels. Sunpower panels, for example, are more than double the cost of a good quality 250 watt one and can only be installed by a Sunpower Partner.
The Samill is warranted for 10 and can be extended to 20. Solax is at the lower end of the market in terms of price.
Price seems a high, but depends on panel type, so I would get another comparison quote here from one of our more localised installers.
I feel one of the best ways of judging is to speak to an installers previous customers near to you. Good installers will give all prospective customers the chance to speak to previous customers to their postcode.
Hope this helps.