Not knowing how far along you are in your four-year program, I am not sure how practical it is to “change horses.” But if you are asking this question, I think that you probably have to seriously look into doing so. In order to be successful in life, you will need to nurture what you love to do, regardless of whether it “fits” into a track that you set for yourself or not.
Perhaps you mentioned marketing and web development because you have a creative bent. I agree with you that marketing and web development are excellent skills to have at this time as these can help just about any kind of business to grow. But from my observations, web developers can get bogged down in that arena and not actively participate in the other aspects of managing the business. I recommend that you ask the career office at your university—if you haven’t already done so—to help you by providing a personality assessment that will tell you whether you have what it takes to run your own company. A desire to do so is important; but you have to be a risk-taker and have characteristics such as persistence/tenacity, a propensity to be persuasive and optimistic, and so forth.
Regarding the tools to launch and run your own company, you could do far worse than a degree in business or business finance with a minor in marketing. But your engineering degree can provide you with an excellent base that you can augment with the business skills that you need. You might continue with your degree and tap into the resources available at your school. Some colleges and universities actually have specific programs to prepare young people to be entrepreneurs with incubator offices right on the campuses. Also, many colleges and universities offer courses in web development that you can take as an elective. Couldn’t hurt to give it a try.