In reply to tommylongfellow:
In Snowdonia, I'd suggest, if a III / IV route is formed enough to climb then it's thick enough to escape.
Your question made me think of the climbs I've done of that grade. It's unlikely that you're led out so far from a resting place that you can't get in adequate protection such as a pair of screws to enable retreat. Most of the Devils Kitchen climbs would be escapable. The Ramp would be tricky once you'd turned the corner, but you'd find a way off Screen, Kitchen, South Gully, etc. Grecian 2000 feels a bit run out but you'd know that from below before you started.
The Snowdon Yr Wyddfa routes would maybe be more reliant on finding sparse rock protection to bolster an escape.
In the Carneddau the objective problems are the onrush of darkness and distance from the road. Escaping the climb is only a minor part of the journey home.
Written advice is less helpful than practical experience. I'd be happy to mentor you on Snowdonia III / IV routes till you get going. All you'd have to do is provide the full winter conditions.