It's disturbing that you can't tell, at 4 months of age, the difference between a Mastiff and a Malinois. It's disturbing that a so-called "breeder" would sell a pup of either breed to an inexperienced owner as both have challenges in their training, mainly to work with their built in desire to protect, but also with the Malinois a very, very, very energetic and high drive dog.
Please, I urge you, get into training classes. These breeds need a firm and knowledgable hand.
If your pup is only 4months old now, in February, then he will only be 8 months old in June. Most advise you will hear about running is not before 18 months or even 24. This is to allow the soft joints of puppy to join up and become solid before you stress them.
My breeder also cautionned against too much swimming on soft joints. LOL, just think, is it harder for you to run in the water? Harder for your dog too. The stresses on joints while swimming are different because swimming is not weight bearing but there is still stress; more like using a resistance band when you exercise.
Your breeder should be available to answer any questions you have, is this the case? Hopefully you were provided with guidelines on how to raise this energetic breed. Please consult with your breeder on the training issues, your plans to run and swim the dog. I find it hard to believe this breed would be sold into a pet home at all. I would expect the breeder to at least require the dog be involved in a sport like fly-ball (I know one who is) or Rally (I know another two who show up at our trials) or agility or competitive obedience. Schutshund (sp?) is an area where Malinois excel.