In 1972, Mader showed that every graph without a 3-connected subgraph is 4-degenerate and thus 5-colorable}. We show that the number 5 of colors can be replaced by 4, which is best possible.

翻译：1972年，Mader证明了每个不含3连通子图的图都是4-退化的，因此是5-可着色的。我们证明颜色数5可被替换为4，且这是最优的。