In reply to Mr-Cowdrey: I'm afraid I'd say your asking the impossible.
A Terra Nova Ultra Quasar is pretty much what you're specifying as soon as you say sub-3kg and genuine 4-season performance. However you are looking at a minimum of perhaps £450 even on sale. Other options that meet that criteria are the Hilldeberg Jannu (more expensive) and MSR Fury (possibly cheaper) although in both cases they are single porch designs. None of them have snow valances which might be an issue for Alpine Winter use but they are a pain the rest of the time and probably best avoided anyway.
However, with an Ultra Quasar you are looking at ultra-light poles, fly and groundsheets which aren't going to be that durable in the long term. [I always use a groundsheet protector and I am on my 2nd flysheet, 2nd set of poles and the inner has been back to TN for repair.] Equally, you will undoubtedly find that the tent (along with the Jannu, Fury etc.) is still overly heavy for the 80% of the time you are camping from Spring through to Autumn. I now haven't used my Ultra Quasar for several years now and generally use a light TNF backpacking tent.
If you are happy to relax the weight limit upward then both the standard Quasar and the Alpkit Kangri
http://www.alpkit.com/tents/kangri are worth considering. Although you will still need to pay £300+ (rather than £400+).
However, I'd probably suggest looking at some quality 3-season tents. First, they are massively better value for money. Second, they will be more suitable for much (although not all) of what you want do.
I personally dislike the 3-4 season rated Trisar linked to above. I've used them extensively recently and whilst I can't fault them in terms of value for money, I find them poorly designed in several aspects, notably their profusion of guylines which seem to hinder rather than help.
One of the best 3-pole semi-geodesic tent currently on the market is the North Face Tadpole 2 DL. For a decent review, have a read of
http://www.cheaptents.com/blog/tadpole2dl/
You've got loads of options but you just need to decide what is more important in terms of weight, durability, size/number of porches, extreme weather performance and ultimately cost.