Can't get this one correct please help. And if you could explain that would be greatly appreciated thanks!

Assume a model with 4 numerical explanatory variables and 1 categorical explanatory variable with 5 category levels. Further assume the model includes the possibility that the relationship between the lst numerical explanatory variable and the response depends on the levels of the categorical variable (include an interaction between the lst numerical variable and the categorical variable). What is the error degrees of freedom for this model if the total number of observations used to estimate this model