  DP-coloring was introduced by Dvo\v{r}\'{a}k and Postle as a generalization
of list coloring. It was originally used to solve a longstanding conjecture by
Borodin, stating that every planar graph without cycles of lengths 4 to 8 is
3-choosable. In this paper, we give three sufficient conditions for a planar
graph to be DP-4-colorable. Actually all the results (Theorem 1.3, 1.4 and 1.7)
are stated in the ``color extendability'' form, and uniformly proved by vertex
identification and discharging method.
