Changing your training program every 4 weeks is very popular.
It’s also one of the dumbest things you can do.
Much of the “gains” you experience in the first few weeks of a new program are neural i.e., your nervous system getting used to the workout and the exercises.
It’s not until after this break-in period that any increases in strength are more likely due to gains in muscle tissue.
By changing your program every 4 weeks you’re essentially re-starting the whole process just before you start seeing meaningful returns.
So how can you keep things fresh while not sabotaging progress?
Rather than make wholesale changes every month, do minor changes as needed to coax progression (ideal) or just provide subtle variety.
You can progress the exercise (go from trap bar deadlift to RDL to conventional DL), or tweak it (hammer curl to supinating DB curl), change the number of reps, change grip width or foot position, even the order of exercises.
The more advanced you are, the more frequently you’ll need tweaking. But total program overhauls should be the LAST THING you do.
I prefer to save those for when life demands it: your schedule changes, you switch gyms, etc.
I’d go so far as to say the longer you can stick with the same base program AND make progress, the better.
Don’t chase novelty, chase results.