It seems to be a universal thing. In every single gym where I've worked, trained, or given seminars, the "4-weeks shuffle" seems to be in effect. In those gyms, personal trainers insist that you change your program every 4 weeks. To do otherwise is to slit the throat of your training progress. Change, or the body gets "used to" the training and it stops adapting.
Let me be blunt for a moment: This is utter and complete bullshit.
The trainers who tell you that are either oblivious to how the body adapts to training (in which case they are incompetent), or trying to take your money by having you buy new programs more often (in which case they are crooks). In either case, it should make you want to look elsewhere for training advice!
Overhauling your program completely every 4 weeks is one of the greatest lies in the world of training. This is how it generally lays itself out:
The first week you perform a new program, your performance is pretty low simply because some of the exercises might feel uncomfortable or your timing might be off. That's normal because you're doing patterns that the body isn't used to doing.
To give you an example of how unfamiliarity might lead to poor performance, I recently went to train with a friend and, just for fun, I decided to follow his routine exactly. His main pressing exercise was the incline bench press. Now I hadn't done a true incline bench press in something like 6 years, so getting 6 reps with 225 pounds turned out to be hard, and I bench press over 405 pounds!
So your numbers aren't really good that first week. By the second week you're more used to the movements and it feels like you're in a groove. As a result the weights go up. Your nervous system is more efficient by the third week; it can better synchronize the muscle fibers involved in the movement and recruit more fast-twitch fibers. As such, your numbers make a fairly big jump on that third week. These, however, are neural adaptations that occur without even a slight gain in muscle mass.
During the fourth week the rapid neural adaptations are pretty much done, so progress slows down a lot. In fact, most people's numbers will not progress on that fourth week. So the trainer changes the program. New exercises and new methods. So what happens? The numbers for the first week are low because everything feels uncomfortable again.
And so the cycle continues. Gains are slow week 2 because you're finding your groove; week 3 sees rapid gains because of neural adaptation; gains slow down or stop during week 4 because neural adaptation stops; and then your trainer sells you a new program at week 5 because "your body has adapted to the program."
The cycle continues until you're tired of your body not changing or you run out of money. The bottom line is that you never really added much muscle mass because the vast majority of your adaptations were neural.
When doing intense lifting, the body is under stress and it will adapt in such a way that if exposed again to the same stress, it will be better prepared to handle it. In doing so, the body will look for the most economical solution. Improving the neural efficiency is much more economical than building new muscle because it doesn't require much actual tissue construction.
Conversely, building muscle is very energy-costly (it costs about 10,000 calories to fuel the processes involved in building one pound of muscle tissue). Not only that, adding muscle tissue also increases the future energy demands of the body. In simple terms the body will absolutely want to avoid building muscle unless it really has to. So, if it has a choice between building new tissue and simply improving neural efficiency, it will opt for the latter 10 out of 10 times.
To start adding more muscle mass you will have to "exhaust" rapid neural adaptations so that when you keep on putting stress on your body, it will be forced to build muscle tissue.
The important point is this: When you hit your first plateau on a new program, that is when you will actually start to build muscle, provided you keep training hard.
If you look at the programs of the biggest and strongest guys, they don't change exercises very often. They might throw in a few sets of a new move here and there for variation, but the meat of their training keeps revolving around the same movements.
Dorian Yates, Ronnie Coleman, Arnold... they stuck with the same movements. When they wanted variation they changed the reps and training methods but they didn't change movements. Through experience, they found that sticking to the basic lifts worked best.
More recently, look at Jim Wendler, Mark Rippetoe and myself. We all have our own keystone lifts that we keep on using year-round. They constitute the foundation of our program and they don't change. We might change sets, reps, and training methods, but we stick to the same lifts. If we change too often, we'd never build as much muscle.
Even bodybuilders like John Meadows sticks to his guns. He uses a slightly greater number of exercises than Wendler, Rippetoe, or me, but he still keeps using much of the same movements year round.
Now, some will be quick to point out that Westside guys in powerlifting use a lot of exercise variation, but by Louie's own admission, the goal is to focus on neural adaptation, and then build muscle with "bodybuilding-type work." The point is, he deliberately changes exercises often to focus on neural adaptations, but if you want to focus on muscle adaptation, you should do the opposite and not change exercises too often.
You may be thinking, "Okay, all I need to do is repeat the exact same thing year round and I'll grow like a weed." Not exactly. Your body still needs variation for continuous growth. If you repeat the exact same workout all the time your body will eventually become accustomed and desensitized to it.
The key is to use smart variation, and smart variation is about keeping the same big money lifts but changing the conditions of performance or the loading schemes.
For example, you could change the sets and reps you're doing, or you could add chains to your lifts for 2-4 weeks, or add more density work (e.g., using 80% of your max and doing as many total reps as possible in 10 minutes). There are many different options, but the key is to keep using the same big compound lifts.