With the rope dart, everything is contextual.
If you are in open conflict, using the rope dart will simply snare a target and yank them toward you and they'll fall to the ground. If you're close enough to them when they've fallen, you can perform a ground kill on them.
If you are anonymous (not in open conflict), you have a few more options. You will do this as long as you remain undetected as you do these actions.
When on the ground, using the rope dart and holding the button down, you will pull them toward you. The longer you hold it, the more you will pull them closer. When the target is close enough, you will perform a kill on them. This works best if you're in a stealth location (e.g., bushes, tall grass, etc.) to reduce the chances of being detected after the kill. If you happen to get detected after the initial pull, you may continue to pull them and will still kill them when close.
When on higher ground, using the rope dart and holding the button down, you will pull them toward you. When they can no longer be pulled anymore (when they're directly underneath you or blocked by an obstruction), you will start choking them. Doing this will grant you a silent kill (not alerting anyone).
When on a beam (i.e., a branch, a rope, etc.), using the rope dart and holding the button down, you will perform the same action if you were on higher ground. If however you move off of the beam while holding the button, you will perform a predator move hanging the target. You will lose one of your rope darts.
I had a very similar experience as you.
The main thing I spent my money on was upgrades for the Aquila.
The towns carried new weapons you could buy, but nothing that was substantially better than what is given to you for free.
Other than that, the only thing to really spend money on was treasure maps & decorations for your house (made through crafting).
Best Answer
It has been quite a while since I played AC:3 and 4, so I can't tell you which buttons to press however, I can tell you what to do.
Aim at someone passing by, the closer to being below you , the better, once they're highlighted you're able to use the action button (I think it's E, however like I said, not 100% sure) and you'll use the rope dart, hitting them and hanging them on the branch where you were.
Edit: When the throw dart icon comes up at the top, press that button and hold S.