Let's get one thing really straight, right from the beginning. You're not going to walk into the gym and just get all the results you want in four weeks. If you have unbelievable lifestyle habits (you eat correctly, you sleep well, stay hydrated, and keep your stress levels down) you will see results in four weeks. Probably not all the ones you want for the rest of your life, but you'll see great results. When you plan out how to get quick results in the gym, there is one HUGE key: consistency and commitment.
The perfect program followed inconsistently is not the best program. The average program followed all the time is the best program.
This is a cliche you'll see thrown around the fitness industry, typically in nutrition coaching. There is no better way to ensure you don't get results than to be inconsistent. If you get into the gym five times one week, the just one the next, then two the following week, then three again, you're not consistent. You're simply not going to see results. I'd rather see you three times every week than the up and down motion of that schedule.
Similarly, if you eat in line with your nutrition plan four days out of every week, then do whatever you want the other three, that isn't consistency, and you won't see results. It would be better to find a less strict plan you can follow seven days a week and slowly take out the things which are not helping you over time. Crawl before you walk, walk before you run.
Imagine yourself in this situation: you've been following your gym's programming consistently for a few months. You're getting stronger, faster, more lean - in short, your fitness is improving.
Then, you're scrolling Instagram one day, and you find what looks like a "fun" workout. You think - oh wow, I really want to try that same program this CrossFit Games athlete is doing. Without thinking about how it fits into the rest of your program, you take it on.
The next day, you check out CompTrain's free online program, and you decide that looks great. Then you see CrossFit Linchpin on Instagram and do their Monday Mash Up. Oh, and Invictus has emailed you their online athlete program which you just have to try. In short, you've followed five different programs in a week. Congratulations, you've just limited your progress.
If you go to a gym that understands programming, or follow an online program from a reputable source, there is a reason certain workouts fall on certain days. With individualized programming, there's a reason you have a rest day when you do, and a reason you have an active recovery day when you do. These things are planned out. One of the fastest ways to make sure you don't see results is to program hop. Just. Don't. Do. It. If you see a workout or two you really want to try, get after it. But don't make it a habit. Stick to the program you're following. That is how to get results quickly in the gym.
Cherry picking extends beyond finding hard workouts and just piecemealing them together. It also happens when you intentionally skip your weaknesses. You hate running, so when it pops up in a workout, you skip that day. Congratulations, you just gave running control over your life. What happens when your kid wants to play with you in the yard? Do you just stand still? How does that feel? Stop running away from your weaknesses. Tackle them head on until they're no longer a weakness. Find the next one and repeat.