What you are experiencing is absolutely normal. You worked on the best idea and you worked it hard. You even delivered revenue from customers and maybe you successfully raised some investor money. However, 4 years later, you're the end of the road. You are hungry for a new idea because you have built up a unique set of entrepreneurial skills that you want to keep using. You're a professional high-energy operator who builds and executes. However, you don't have anything as good as the last business because starting is hard. You know that a startup requires commitment and conviction. However, to get that fire again, for a new business, is a drag. Your startup left you out of commitment and conviction for your industry and with even less fight in your spirit for another run.

My advice: Go back to the scene of the crime and open up an investigation while the trail is still warm and you have some consulting gig to keep cash in your pocket.

Your startup worked for 4 years. I would recommend you go back to your industry and meditate on why you were convicted and why you were committed. Why did you succeed? What did you do better than the competition? Why did you stop succeeding? What about your customers you really loved? Which customers did you really hate? What was the magic of the business that others couldn't repeat as easily as you. In those answers may possibly lie the new startup that you want to launch that will CRUSH the COMPETITION. In a careful poring of the 4 years are the seeds for a new and possibly even more successful company. Take your time because looking at the failure will be painful.