Basically, there is a well-known scheme of the three primary constraints in the project management: scope, cost and time. Also know as iron triangle. Well, it means that a good project must balance accurately between these three limitations.
For example, if you failed in the time estimation you’d have to increase the costs or decrease the scope of the project. If you don’t estimate the scope properly it will cost you either money or time (or both). Finally, if you don’t meet the deadlines, you’ll have to reduce the planned scope and your spendings will also rise. It’s is very approx description. You’d better read something precisely on topic, like this paper. But I haven’t found here anything particularly on quadruple scheme.
In a nutshell, there is an idea that this triangle must be expanded to the rhombus, in the angles of which we see already known the scope, cost and time and the fourth constraint - quality. For what? The idea is that if we make a project that meet the requirements of schedule, budget and scope, there are still no guarantees that the product will be qualitative. However, it is the core indicator of a good product.