In Isaiah 26 verse 4 why is Jehovah used instead of Yahweh in the KJV?
Isaiah 26:4 Trust ye in the LORD for ever: for in the LORD JEHOVAH is everlasting strength
The four-character written name of God, YHWH, is known as the Tetragrammaton. Note that Hebrew is written without vowels. The Jews ceased to articulate the name of God and the pronunciation was lost. Go here for an in depth explanation: https://en.wikipedia.org/wiki/Tetragrammaton
The (modern) scholarly consensus is that the original pronunciation of the Tetragrammaton was Yahweh. The New Testament writers never use the name of God, but substituted the ordinary Greek terms "God" and "Lord". For example:
This is a quotation from Deuteronomy 6:16 where the name of God originally appeared.
I hope you find this helpful.
God bless,
Glenn