At 13,000 feet of altitude (3963 metres) there is almost 40% less oxygen in the air!!
I am full of admiration for your ability to ride even after some acclimatisation.
I rode uphill at a measly 1500m on an mtb and I thought I was going to lose a lung.
NB: A friend of mine hiked around the world when he was in his late 20's. He discovered in a mountain pass in the Himalayas that he could not get an erection at that altitude (15,000 feet IIRC). He'd copped off with a New Zealand nurse and he was very embarrassed. She was so understanding that he married her. Normal service was resumed when they descended the other side.