An epidemiologist is building a model to predict the probability of catching COVID. She has data on people's mask use (X1), vaccine status (X2), sex (X3) and age (x4). To test whether the addition of these four variables significantly increase the fit of a predictive model, she calculates the difference in deviance of the original model and the new model. What is the critical value of chi-square to test the null hypothesis that these variables do not increase model fit (a = .05)