| Succession: | Duke of Saxe-Coburg-Saalfeld |
| Reign: | 6 August 1699 – 17 February 1729 |
| Successor: | Christian Ernest |
| Succession1: | Duke of Saxe-Saalfeld |
| Reign1: | 26 March 1675 – 17 February 1729 |
| Predecessor1: | New creation |
| Succession2: | Duke of Saxe-Coburg |
| Reign2: | 6 August 1699 – 17 February 1729 |
| Predecessor2: | Albert V |
| Spouse: | Sophie Hedwig of Saxe-Merseburg Charlotte Johanna of Waldeck-Wildungen |
| Issue: | Princess Christiane Sophie Christian Ernest Princess Charlotte Wilhelmine Prince Wilhelm Frederick Prince Karl Ernst Princess Sophia Wilhelmina Princess Henriette Albertine Princess Louise Emilie Princess Charlotte Francis Josias Princess Henriette Albertine |
| House: | Wettin |
| Father: | Ernest I, Duke of Saxe-Gotha |
| Mother: | Elisabeth Sophie of Saxe-Altenburg |
| Birth Date: | 22 August 1658 |
| Birth Place: | Gotha, Saxe-Gotha, Holy Roman Empire |
| Death Place: | Saalfeld, Saxe-Coburg-Saalfeld, Holy Roman Empire |
| Religion: | Lutheran |
John Ernest IV (German: Johann Ernst; 22 August 1658 in Gotha – 17 February 1729 in Saalfeld) 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.
After the death of his father in 1675, Johann Ernest initially governed the duchy of Saxe-Gotha-Altenburg, jointly with his six older brothers, as set out in their father's will. 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. As he was the youngest, he kept the smallest portion of the lands.
Johann Ernest and his brother Ernest soon found themselves financially overstretched as a result of the partition (the income of their eldest brother, Frederick, far exceeded the income of Johann Ernest), and they both made a protest. 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. During these years, Johann Ernest took possession of Coburg (in 1699), Römhild and 5/12 of Themar (in 1714).
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. His descendants retained Coburg. The decision was generally accepted, most importantly by the descendants of his older brother Bernhard, who also had a claim to Coburg.
In Merseburg on 18 February 1680, Johann Ernst married firstly Sophie Hedwig of Saxe-Merseburg, a daughter of Christian I, Duke of Saxe-Merseburg. They had five children:
In Maastricht on 2 December 1690 Johann Ernst married secondly Charlotte Johanna of Waldeck-Wildungen (b. Arolsen, 13 December 1664, d. Hildburghausen, 1 February 1699). They had eight children: