Okay, a wider pot will lead to additional boil loss, so that's good for you to know for the future. The best thing about your setup is that you're using a 15 gallon kettle. That means that you can do more sparging than most people do for a 5 gallon batch, which will allow you to increase efficiency and start with a larger amount of wort as you begin the boil.
Since you ended up at 4 gallons, now you know you need to start a 90 minute boil in your setup with at least 8.25 gallons. As another poster said, you'll learn from experience, and hopefully you'll have some fun along the way.