Sort the following events to reflect the order in which they typically take place during the reprogramming of fibroblasts to induced pluripotent stem cells. Your answer would be a four-letter string composed of letters A to D only, e.g. ADBC.
(A) Sustained endogenous expression of Oct4
(B) Gain of embryonic marker protein
(C) Loss of fibroblast-specific marker protein
(D) OSKM overexpression