It is unlikely that you will lose muscle mass while going on vacation for 4 days. Your body needs more than 4 days of rest in order to start breaking down muscle tissue. Therefore, if you are taking a short break from working out and eating nutritiously during your vacation, it is unlikely that you will experience any significant loss of muscle mass.
However, it is important to note that if your vacation involves a lot of sitting or lying down and not moving around much, then you may experience some minor muscle atrophy due to disuse. To prevent this from happening, make sure to incorporate some light physical activity into your daily routine while on vacation such as walking or swimming. Additionally, make sure to eat plenty of protein-rich foods to help maintain your existing muscle mass.