The probability 1 in 4,000,000 will result in an acident.
over a life time an average driver takes 50,000 trips.
what is the probability of an accident over a lifetime
That would be one MINUS the probabilty of NOT having an accident 50,000 trips in a row. That is equal to
P = 1 - (1/4*10^6)^50,000
= 1 - (0.99999975)^5*10^4 = 1 - .9876
= 0.0124 , or 1 in 80.
This must refer to FATAL accidents. Most drivers have at least one accident during their lifetime.
The 1 in 4 million refers to the probability of a fatality in a single trip. You did not state all the facts of the problem.