Can you really just look at the cumulative amount of radiation? Does the intensity not matter? If you compressed those 4 days' worth of exposure into, say, 1 second, would that not have any different effect?
The predominant wisdom is the linear no threshold model - https://en.m.wikipedia.org/wiki/Linear_no-threshold_model
and the counter argument, the radiation hormesis hypothesis - https://en.m.wikipedia.org/wiki/Radiation_hormesis