Hi, I'm not sure if you're aware of this but Facebook has a spam filter built into it. If you sent 4 emails to people that you don't know, they most likely ended up in that folder (which most people don't know about and never check). So most likely no one ever saw your message!

Also, 4 emails is also far too small a number to judge anything from. But more importantly, validation of your idea is usually an interactive exercise, and not something that you want to leave to an email.

If I were you, I would go out to the next and nearest food truck rally, and hang out there. Most food trucks are run by the owners, and there are always lulls in the day when they are not doing anything. That's when you should introduce yourself, say something very complimentary about their food, and then ask them if they have the problem that your idea solves. If they do, then, that gives you the opening to talk about your product/service. What you'll most likely find is that shall not only get feedback space on if your idea is good or not, but also refinements that will make it better.

And don't ask just a few… Make it your project to hit at least 80% of them there. After that you will have a better idea of what are your idea will work or not.

Good luck!