Cut the pork tenderloin into 4-5 evenly sized pieces and place in gallon ziplock bag.
Pour in 3 tablespoons of the char siu sauce and zip up the back securely. Knead the sauce into the tenderloin and put in the fridge for at least 4 hours and up to 24 hours.
Heat oven to 375 F.
Line a baking sheet with foil and place a rack on top of it.
Place the tenderloin pieces on top of the foil and dab on the remaining sauce on each side of the meat.
Bake for 25 minutes on one side.
Mix the remaining 1 tablespoon of sauce, honey and water together into a small bowl.
Dab on to the meat and flip it over, dab on this side as well. Don't try to spread it, just dab it so it goes on thicker.
Put the meat back in and cook for another 20-25 minutes, or until a thermometer reads 165 degrees.