בֶּן־שְׁלֹשִׁ֥ים שָׁנָ֛ה דָּוִ֖ד בְּמָלְכ֑וֹ אַרְבָּעִ֥ים שָׁנָ֖ה מָלָֽךְ׃
David was thirty years old when he began to reign, and he reigned forty years.
Read Kabbalistic (mystical) interpretations of II Samuel 5:4: Zohar and classical Jewish mystical sources, free online.