It sounds like you are not quite clear on how this "bonus" even works. In a comment you say the bonus is given quartly at 40% regardless of personal contribution? That in itself is not clear (to me) - 40% of what? of base salary for the quarter?
It is indeed hard for anyone to give much advice as all companies are different. But I do not advocate listening to what they TELL you, you really should have the terms of the bonus in writing, especially if it's going to be a substantial portion of your compensation AND reduces your actual salary.
You should be able to find the straight truth about the nature and possible conditions of this bonus if you just look at the employment contract/offer letter. If the contract says "employee will be paid bonus every x months at y amount, regardless of performance", as you have been told, you might actually be fine. But remember: the contract is all you're going to have going forward if you take the job, so you should take it at face value and believe what it says, independent of what anyone tells you verbally. It's that simple.
You don't include any location information but just in case: I had honestly never seen a scheme like this where the offer is presented with a "salary" that is rolled up with a fully discretionary bonus until I moved to Germany (I'm from the US). Provided this isn't a christmas or vacation bonus (these are special), from what I have seen, the "bonus" was never a carrot, it was always a stick. I can't help but wonder if you are in Germany due to the "DE" in your username, so if you are, please be aware that you might encounter lots of mobbing at bonus time under this kind of scheme (where they are basically holding money you NEED hostage).