In Bloons Tower Defense 4 (a.k.a. BTD4) you pretty much just pop balloons for the whole game so here's some tips about what to do to get to the big levels:
1. Towers When you start a new game or you're just starting to actually play you place weak towers right? That's why at the beginning you start with the weak towers and buid up to the big ones because if you start out with the big tough towers you won't be able to keep up with the prices. So main thing start with the weak towers, build up to the big towers.
2.Tower placing If you have the strategy or idea of what tours you're going to use then you have to think about where you're going to place them. The best place to put the towers is where the would pop the most balloons. Like a cannon in the corner of a turn will do alot of area damage. So main thing think about where you are going to place your towers based on whatb they do.
3.Upgrades When you buy a tower they always have upgrades right? That's why you want to rank up to unlock the upgrades. But spend your money carefully. If you've unlocked a new ugrade for a tower try it out in sandbox mode ( if you've unlocked it) to strategize. So main thing use your upgrades wisely!
4.levels As you progress through the game you get to higher levels so you want to buiild a strategy to get to the high levels. What i'm saying is practice, practice, practice to build the best strategy possible. Once you've achieved that just try to keep going, and going, and going! So main thing build a strategy to get to the higher levels!
5. Mochi Upgrades You probably already know that there are Mochi upgrades. My perspective of them: they ruin the fun. I don't use them because I like to play fair. I would only use them if you beat everysingle track on easy, medium, and hard. So main thing only use Mochi upgrades if you've COMPLETELY beaten the game.
Those are the tips that I've got so I hope they help with your gaming on BTD4!