Let's be realistic... It takes quite a large concentration of fuel vapor to cause an ignition.
I have an Atomic 4 and I have never had any fuel fumes issue.
Unless your engine and fuel lines are in terrible condition, which you would be obviously fully aware of, the odds of anything catastrophic happening to your fuel lines or carburator between the time you shut the engine down and the time your crew fell overboard are simply minuscule in my humble opinion.
I agree with the previous answers that practicing and perfecting MOB recovery under sail is a must, but if you really have to start the A4, I would not hesitate.
Just my opinion as a A4 owner.
O'Day 30 - Stuart, FL
Oceanis 411 - South of France