Po(m;x)=e^-m * m^x/x! symbolises the Poisson probability of x events for a mean m events.
When x=3 Po(m;3)=e^-m * m^3/6 and when x=4 Po(m;4)=e^m * m^4/24.
Po(m;3)=(2/3)Po(m;4), so m^3/6=(2/3)m^4/24, 1/6=m/36 and m, the mean=36/6=6.
In a Poisson distribution the mean and variance are the same, and SD=√variance, so SD=√6=2.4495 approx.