If being a Sergeant is your goal in your first four years, and you are in the Marines, there are two very important things that are not entirely that much of an indicator of performance, but are extremely important to your Marine Corps destiny nonetheless.
Before that, you need to know how the Marines promote their enlisted. The E-1 through E-3, the first three ranks are awarded automatically after certain periods of time. The Marines promote their E-4s (Corporals) and E-5s (Sergeants) based on something called a cutting score. Your cutting score is the number of points you have earned based on your time in rank, your time in service, your conduct at your job, your accuracy with the rifle, and physical fitness, as well as if you have done any additional training in the form of MCIs. Since time in service and time in rank always go up, assuming you don't drop in your scores in other things, you will eventually get a higher score every month and more every quarter.
1) The job you choose is extremely important in promotions.
Each MOS (Military Occupational Specialty) or job, puts out different scores from quarter to quarter. If your score is above the score posted or grows in that amount of time, congratulations, you get that extra chevron. You will probably already have around 1200 points as soon as you are promoted and need to gain points before your next one. Some MOSs need a very low number, around 1400, and others need around 2100, and then some just close out completely. I worked in a job that wasn't incredibly hard and got promoted at a bit under 1700, as I remember. It has been a while, I'm not sure. Then the score dropped, and my dumb little buddy got promoted at 1550 a month later. Didn't bother me a bit...
2) The best way that you can manipulate your score is by working on pull ups.
Pull-up?! You mean that one of the most advanced military fighting forces places a disproportionately important amount of weight on the selection of its leadership with pull-ups? Yes.
The reason for this is the physical fitness part of the cutting score. In all, the physical fitness part accounts for 300 points of the cutting score. The PFT (physical fitness test) has three events, each with a max of 100 points: sit-ups, pull-ups, and a three mile run. The sit-ups are easy to get a max of one hundred. The run awards 100 points for running your three miles in under eighteen minutes. It only takes away 1 point for every 10 seconds after that. So you only lose 6 points for every minute after 18. Basically you can fight genetics for miles and miles to achieve the perfect 18, but the smarter way is to work on pull ups. You get your 100 if you can do 20 in a row. This is a bit extreme since most people never set themselves to it, but is very possible to achieve with a few months of training. The second time I went to Iraq I set myself up doing pyramids. The pyramid technique works best. You do one pull up then rest for a minute, then two pull ups then rest for a minute, then three ... all the way until you can't do anymore. Then you work your way down three, then two, then one, and that's it for a few hours. I went from doing about eight for most my career to doing twenty in a few months. I remember the night when I was in Iraq outside the gym. I hopped up on the bar set to do however many I can. Was sort of keeping count while thinking about all the other things in the world when I tired out. Dropped down and then it hit me ... I just got 21. For Marines, that is a big deal.
Now as far as it goes, most everything else is pretty hard to avoid. Your performance scores are hard to effect, time is only changed with time and you can't change your job, if you don't shoot well the first time, you can't really train your way up to the point where you can. A jump of 60 points on the cutting score definitely helped get me over the hump.
3) Do your friggin MCIs. They are simple tests over some stuff that you should know already. All Marines know what they are, and if you decide to go, you will get the chance to do them. They're optional, but worth 100 points on your cutting score, and the only reason you don't do them is laziness.
If you get a job with a good cutting score, and keep with your pull-ups, it might prove to be possible to make Sergeant in four years. There are a lot of us who did, but no matter what I have made you believe, it was still hard. These things are just the things that are most easily improved if you work at it. There is much that is required just to maintain that cutting score, and most of it is the unpleasantness that goes with being a day to day Marine for four straight years.More questions on US Marine Corps: