A Bernese puppy can gain a kilo a week during peak growth. The skeleton is soft and the growth plates are open, which means the exercise choices you make in the first year matter for the rest of your dog life.
A useful guide is around five minutes of structured walking per month of age, twice a day, so roughly twenty minutes twice daily at four months. Free play on soft ground in short bursts is better than long outings.
Limit repetitive impact: no stairs where avoidable, no jumping in and out of cars, no long runs on pavement, and no chasing a ball until they are physically mature. Non-slip flooring indoors helps more than most owners expect.
Keep your puppy lean. Extra weight on developing joints is the single most controllable risk factor, and a slightly lean large-breed puppy is a healthier one. Feed a large-breed formula and measure it rather than guessing.