Trey Cook has said that both he and Fredrik Ericsson have decided not to try for the summit today. They are both exhausted after battling the difficult climbing conditions, the weather, and the altitude that they are at.
The plan had to be leave for the summit at 10pm on the 26th. But they only arrived on the shoulder at 8pm. This would not have been enough time to make camp and brew liquids for the next day.
So exhausted where they that C4 was setup well below where it is normally. They were over an hour from where it is normally on the shoulder. Had they gone for the the summit this would have further added to their fatigue on the last day.
The snow between C3 and C4 was not very deep. There was thin coverings of ice and snow on the rocks. This made the climb more technical. When they placed their feet sometimes the rock crumbled and they had to make sure that they kept their balance. The wind has also been a factor. It has been blowing hard enough to make climbing difficult.
Closer to C4 it has been reported that there has been waist deep snow. This would further add to their exhaustion. The wind has subsided though.
They might do a recon further up today and then head back down the mountain. This might delay their arrival at base camp by a night.