The general consensus on the forum is that in the interest of food safety you shouldn't inject the butt until the outside temperature of the butt has reached 135 degrees.The marinade and needle should also be sterilized before injection. By injecting the night before you are introducing bacteria into the muscle of the meat and it is highly unlikely that you will get internal temperatures above 135 degrees in 4 hours to satisfy the 4 hour rule. This has been debated quite a bit on the forum and we the moderators of the forum have the responsibility of ensuring that new smokers are aware of proper food safety procedures. We also are not the food police and if you wish to inject the night before no one is going to stop you. Just be aware that if serving to an at risk population you can make them ill.
I suggest you do a tremendous rub the night before and if you wish to inject do it with sterile marinade and needle after the meat has been in a hot smoker for at least one hour. We also do not recommend allowing the meat to sit out of refrigeration prior to smoking.
Good luck and enjoy your smoke