Continental GP5000 or 4 Season for all year riding

I use both GP4000 and GP5000, I would say the GP4000sii were a perfect all year round tyre and a good race tyre, the GP5000 is a better race tyre but not quite so long lasting and Im not sure if I would ride them in winter. Fortunately I have lots of spare GP4000's as they seem to be unavailable now. The 4 Season I tried and found lack lustre and wont use them again.

Edit: I see GP4000sII are still available, I'd grab some of those if it were me.

Hutchinson Fusion 5 all season tubeless,  I've just done a second winter on the same pair with no flats at all despite a lot of km on wet and gritty roads. They ride very nicely and can be had for not that much dinero.

I ran 4 Season for a few winters and was always very pleased with their durability but less so with the grip in the wet. I ride all the absolutely shitiest little back lanes with grass growing up the middle and slivers of flint everywhere  (especially this winter). When I got tubeless compatible rims I switched to Scwalbe Ones which I loved for the speed and traction but they became quite fragile after about 8/900 miles. 

This year I switched to GP5000 and I have to say I was super pleased with the comfort, speed and grip. I've heard a couple of small puntures self seal but they showed no wear until 500 miles when a few cuts began to appear. Got my first cut that wouldnt seal at 850 miles and I would say that the rear now shows quite a bit of wear. I've booted it from the inside and I reckon the rear will do another 500 miles the front a lot more. For me that will be acceptable as long as there are no further punctures. I'll take that limited rear wear over superb speed, comfort and grip.

I stopped riding "winter tyres" a few years ago and have much more fun! Vittoria Corsa 25mm on my "summer bike" and Shwalbe Ones 28mm on my " winter". Run them at slightly lower pressure for increased grip dependant on conditions

Personally, I really don't rate any of the Continental bike tyres I've ever used, in terms of durability. I wouldn't want to run any of them all year round. I've found Hutchinson equinox to be a big surprise ( in a good way ) and a lot of the vittorias I've used have also been pretty solid, but not at the cost of ride killing performance compromises.

Unless you're going for the GP5000's I would choose something else above the 4Season. I've found the 4Season sluggish, no better than my other tyres for puncture resistance and not especially grippy in the wet. Not a very supple tyre either.

I like all the Hutchinson Fusion tyres in the 11storm compound and have All Season, Performance and Galaktik variants across my bikes.

Treat yourself. Go straight to the 5ks. 

Grand Prix GT is a bit racier than the 4S; it uses the 5000 compound but with the 4S puncture guard. It's a great tyre. I commute on it in London and can't remember my last puncture. Grip is good. I'm a tyre snob as well (Veloflex tubs on my best bike)

It depends really. If you value going fast then the GP5000 tubeless tyres are brilliant, however I'd personally use different tyres in the winter. The grip on them is better than 4 seasons in my experience. 

The 4 Seasons are noticeably slower but if puncture protection is a bigger priority than speed then I'd use them instead. 

With regards to the other points raised, I've never had any issues with getting other tubeless tyres onto Mavic UST rims, in fact they're some of the easiest rims around to get tyres onto. Mavic tubeless tyres are made by Hutchinson who also make tyres for several other brands as well as their own ones.

The GP5000's are tough to fit initially but like all tyres, once they've been used they stretch a bit and they're not issue to remove and refit.

I've had a tubeless GP5000 28mm on my rear wheel through the winter and it's been flawless. I've just recently replaced it (something like 8 months wear) and put one on the front too.

Four Seasons all year long!  Which is kind of the point, I guess.