You'll get it!! I've done maybe 12 lessons and we started posting trot right away. It came to me fairly easily (although I'm still posting a bit high and still need a lot of work on it in general).
One thing I find really helpful is counting out your beats OUT LOUD. So 1-2-1-2 or up-down-up-down as you rise and fall. My posting also improved when I did it with my hands on my hips (no reins) in the third or fourth lesson - much less scary than it sounds. Not having to worry about my hands allowed me to really focus on the rhythm and also my balance, as well as using my eyes and body to steer the horse where I wanted him to go. It also helped me to keep the horse going long enough that I could really get a feel for it. Then once I got that, we added the reins back in. I'm getting better at keeping my hands low and quiet.
I will say that the horse I ride is bouncy enough that it makes the posting trot feel "legit." I rode a horse in one lesson who was extremely smooth. It made it very hard for me to post because I couldn't feel the rhythm as much and I hated his saddle in a big way, which made me reluctant to post on top of it because it was very painful in my nether-regions.
Anyway, long story short...I feel your pain. Riding definitely seems to work in a random pattern of exciting and frustrating lessons. We've ALL been there. But once it clicks, it's just a matter of gaining the muscle memory so you can do it again and again. It feels great once you get it! Keep working on it. Riding wouldn't be any fun if we were all naturally good at it - the challenge is what keeps us coming back for more.
Posting is one of those literal practice-makes-perfect kind of things. In fact, riding in general is. It's extremely difficult at first because 1. you don't have the proper leg strength and 2. it's overall an unnatural feeling. But don't worry. In time your legs will build up their muscle and you'll get the hang of it. Before you know it posting will become a second nature.
When I first started posting I got the rhythm of it pretty easily but it took me a few lessons to make it look somewhat pretty without bouncing around like a crazy fool. And even then it wasn't that great. Heck, I STILL get bouncy and ugly when posting the trot at times, and I still find myself looking down to make sure I have the correct diagonal sometimes (although my new lesson instructor complains if I look now)! Not to mention I was very, very painfully sore when I started the trot.
I just moved from doing a year of Western riding to English riding and feel like a clutz all over again. I'm still getting used to the position, the more powerful/bigger gaits, and the steering! I am having trouble steering the horse! You'd thing I would have gotten it by now but it's a different horse and a different discipline.
It's only lesson 4 for you. You have a long way to go and if you want it, you'll get it.