I agree with other HNers who are suggesting taking a break. It really does look like burn-out. You could look at how you viewed the startup when you joined.

I agree with other HNers who are suggesting taking a break. It really does look like burn-out. You could look at how you viewed the startup when you joined. What excited you, how you felt that you were contributing to the vision. Then look at how and when that changed. Did the management change? Were a couple of unrealistic deadlines missed? After 4 years have you had a salary increase? Is it the still the same product or have there been pivots? Has the market not materialised? How about further investment? In 4 years things have probably changed and maybe you have ignored the signs that are now troubling you subconsciously.