My sleeping bag is a Mountain Equipment Xero 550 which I bought in February 2008; it’s quite light for a three season bag and very comfortable. I'd like to add a 4 season bag to my winter shopping list, but I really can’t justify the cost of owning more than one sleeping bag.
Using the same bag for all seasons does have its downsides, in the summer you’re carrying unnecessary weight, and it can become too hot, although the temperature is easily controlled with the two way zip. In the winter it can be quite cold, so I make up for the lack of down by wearing my trousers and either a fleece or my Patagonia Nano Puff jacket.
The mattress I've been using this summer is a NeoAir short, at 278g its very light, but for the winter I prefer a full length mat, so I’ll be reverting to my Prolite (472g). If it looks like being really cold, then I’ll use my old green Trecklite, which I’ve had for many years. It may be quite heavy at 823g, but even on snow is toasty warm.
I don’t cook in the hills; I only boil water, either for brews, dehydrated meals or the occasional cuppa soup. This means my summer cooking gear should be perfectly adequate for the colder weather, the Optimus Crux Light stove (74g) works well with a Tibetan 550ml Titanium mug (87g), and boils quite quickly. This set up used to wobble alarmingly, before I solved the problem buy fitting a Primus gas canister feet (28g), which is quite stable on most ground. The one item missing from my camp kitchen is a windshield, so I intend to custom build one, from a disposable foil tray before my next trip.