John Ernest IV (German: Johann Ernst; 22 August 1658–17 February 1729) was a reigning duke of Saxe-Coburg-Saalfeld.

He was the tenth but seventh surviving son of Ernest I, Duke of Saxe-Gotha and Elisabeth Sophie of Saxe-Altenburg.

However, in 1680, the brothers concluded a treaty dividing the paternal lands and Johann Ernest became duke of Saxe-Saalfeld, with the towns of Gräfenthal, Probstzella and Pössneck.

Over the following years, the controversy continued and increased, as their older brothers Albert of Saxe-Coburg, Henry of Saxe-Römhild and Christian of Saxe-Eisenberg died without male heirs.

The "Coburg-Eisenberg-Römhilder Erbstreit" was finally resolved (after repeated intervention and arbitration by the emperor) in 1735, six years after the death of Johann Ernest.