How do we impute annual_income here ?

Before predicting the recovery rate in the Expected Loss model , the annual income variable has to be treated as it has null values. The two variable (mths_since_last_delinq,mths_since_last_record ) have been treated but
annual_inc variable has not been treated for missing  values. There are 4 missing values in the annual income variable. Shall we impute them to zero?
I have also tried imputing them to median but I am getting an error. Please advise for it.