At only four days of riding, posting will be tough to nail down right away. Don't worry, it'll come with time and practice. I guarantee there will be a lightbulb moment, probably pretty soon, and you'll wonder how posting was ever tricky at all.
Learning to post is easier on a horse with some natural bounce that is moving out properly. If you're riding a horse with a very pokey, smooth trot, or one that prefers to do more of a western-type jog, you'll end up working harder than the horse.
A visualization that I found really helpful is to imagine a giant spring hooked to your belt buckle, and stretching at a 45 degree angle up into the sky in front of you. Imagine the spring directing the movement of the posting action. I find that, armed with the "spring image," it's easier to get the right kind of forward and back element to the movement, rather than trying to go straight up and down, or to "stand and sit," which is much harder, works against the horse's movement, and gets the rider more reliant on their feet and stirrups than they should be.
Ditto on the lunge line lessons -- with you on the lunge, all you have to worry about is posting; the instructor will take responsibility for keeping the horse moving forward, and you won't have the temptation to use the reins for leverage. Although at this early stage just getting the movement down and correct is most important, eventually you'll need to post on the correct diagonal. There are real biomechanical reasons for this, but for now it'll be easiest to remember to "rise and fall with the leg on the wall." Your instructor will cover diagonals in proper detail when you're ready for that little wrinkle.
You may want to try a few strengthening exercises between rides. Look at exercises that target upper thigh and core muscles. Also, riding for stretches on the flat in two-point position can be great for your "posting muscles" and overall position. Provided that your saddle fits you well and the stirrups are properly adjusted, correct two-point forces you to adopt a more correct lower leg position to stay balanced and stable -- focus on maintaining that nice leg position when you sit back down, and pop up into two-point at random intervals to check yourself. If you have to shuffle and readjust your position before you can get into two-point, something has slipped in your leg position. Perfect practice makes perfect.
Good luck, and have fun!!