The 4kW was all about DNO permission, you need approval to go over 16A export. That's still true now, doing a G99 application is more paperwork than a G98 and some companies can't be bothered with either the paperwork or explaining it to users.Most people don't have space for a 6kw system. The average is still a 4kw system because that's what will fit on the majority of solar suitable roofs. Energy Saving Trust has average cost of a 4kw system between 6k and £7k with a payback period of 20+ years. You will get a better payback if you are home during the day - or part of the day. In the 20 years you'll save about £200-£300 per year, but that doesn't account for what you might do with the money otherwise. Battery technology will change things for solar. When we are able to store energy efficiently and use it later - when it's needed - the figures change.
PS, not so long ago the weak link in the system was the inverter, which was an expensive item and typically lasted 4 or 5 years. These days, for a bit more money, you can get a 25 year warranty. I wonder how many people pay the extra though?
If you are getting quotes for 6-7k for a 4kW system then the numbers will never work out for you with sane electricity prices. Either you need better quotes or your property is too awkward for Solar to make sense.
Lots of small roofs, shading, awkward access, dodgy roof structure can all make it impractical to get Solar. But if you've got a nice South facing roof, or better yet East and West facing roofs then it's worth looking into.