I recently rode a series of tough climbing rides on the order of magnitude of (but a wee bit easier than) the Death Ride. 30-40% of my time was spent in HR zone 4 on these rides. The Mulholland Challenge was probably the closest to the Death Ride in difficulty, 124 miles/13000 ft climbing. That's the one in which I spent 40% of my time in zone 4.

So at least from one person's perspective (mine), I would not be at all afraid to spend substantial time in zone 4 (obviously). I do ride a fair number of hypoxic type intervals and I train pretty seriously with a coach. But I'm not an extraordinary athlete or anything. Yes, I'm tired at the end of the rides but still functional. I assume if I can do this, so can you. But honestly have no idea if this is universally doable or just how I happen to ride.