By a well-known theorem of Thomassen and a planar graph depicted by Voigt, we
know that every planar graph is 5-choosable, and the bound is tight. In 1999,
Lam, Xu and Liu reduced 5 to 4 on C4​-free planar graphs. In the paper,
by applying the famous Combinatorial Nullstellensatz, we design an effective
algorithm to deal with list coloring problems. At the same time, we prove that
a planar graph G is 4-choosable if any two 4-cycles having distance at
least 5 in G, which extends the result of Lam et al