Nobody in the company should be irreplaceable, otherwise you have a single point of failure.
On a more practical side: don't work on your vacation (and I mean this in general - like ever) or it doesn't make sense to even go on one. If you must, you can try shorter breaks first so you get confident that the world is still turning once you get back. And do not answer phone calls during the breaks!
This, and make it clear in various forms (email, several personal meetings, via wife, via friends, et cetera) that any outcome of the state of the company after return is okay. It's the only way to ensure the vacation is effective: vacation with guilt is having a lot less effective vacation. "Freedom" (from the company) must be felt.
And if it doesn't, then hell, it wasn't going to survive with you either! So there's no need to let those thoughts consume you on your vacation.
1. full recharge
2. peace of mind knowing that your company can survive without you
3. it will help you think ahead: you will realize that your immediate work is not the most important of your tasks. You need to focus on the future of your product rather than on operational issues.
3. less guilt about taking vacation
4. your employees / cofounder will feel empowered that you let them in charge for a little while
5. your employees will feel less guilty taking vacations themselves
On the other hand, the drawbacks of taking a full week off are:
6. 1 week of funding 'lost' - yes, but you will realize that you will quickly make up for the time you took off.
7. possible failures: yes, although you have 3 guys in your staff, they should be able to dig into issues, probably not as fast as you can, but, hey, you won't be debugging forever... So they've got to learn as well.
8. annoyed customers: yes, well, your product is not perfect yet, so that's the way it's going to be for that week.
In fact, I think what's important to realize is that if you find yourself in this situation, it means that your company made some mistakes somewhere. You guys made some trade-offs in favor of quality or time to market and at the expense of personal happiness. And now it's payback time. That "overwork" debt the company took is no longer sustainable.
I think the ability to go hands off is way underrated. It is a true skill that you need to develop. For your own sake, but also for the sake of your company.