Potty training a Mastiff puppy is not much different than potty training any other puppy. You're going to need to show your puppy where he should be eliminating and reward him for recognizing it and then doing so. Mastiffs are eager to please. So your Mastiff puppy is eager to learn from you. Approach potty training with enthusiasm and high energy so it's a task he's happy to do.
Having a large Mastiff in your house also means potentially having large amounts of urine in your home. Potty training him early on through positive reinforcement and reminders while building this new habit will be imperative to keep a clean house and having a well-trained dog.
Your Mastiff puppy will love the idea of going outside to go potty because he gets to get some exercise while he's out there. End this outdoor time rewarding him with a little bit of play time so he can burn off some of that excess energy he has. Potty training your Mastiff puppy will take some time and commitment. But most of all, potty training is about repetition and knowing the right times to show your Mastiff where he should be going potty. Potty training your Mastiff is certainly easier if you can be home a lot during the day, especially during these early times in your relationship with your Mastiff. House training may take up to 6 months to perfect, but you can start expecting your Mastiff puppy to begin learning as early as 8 to 12 weeks of age.
Potty training your Mastiff puppy will require lots of tasty treats to keep him motivated and remind him to make good choices when he needs to go. You will also need to be cognizant of his schedule and the signs, and cues he gives to let you know he needs to go potty. If it all possible, having someone around during the day certainly helps. If not, keep in mind a puppy can typically only hold his bladder for about 1 hour for every month he is old. So your 4-month-old Mastiff could probably hold it for about 4 hours if you're out of the house. You can also teach your Mastiff puppy to tell you he needs to go potty using a bell or a knock on the door. Make potty training fun and rewarding.