Civil Engineering is based upon the design and processes of statics.
By statics, I am talking about things that are designed not to move (like bridges, roads, walkways, etc), and are supposed to be strong and quite reliable over time.
Also, it can entail management, design and abiding of regulations of infrastructure. By this I mean as in places where people work, live or play.
In terms of what the actual study you do consists of, its not necessarily all 4U maths.
The most useful parts of 4U maths for you will be Mechanics, Volumes, Conics (in higher years definitely as every traffic intersection is based off conics lol fun fact) and integration. I'm guessing you will have to learn a lot about the logistics of regulations, knowing how to make designs and design plans that follow these logistics.
Also, there will be a major component of mechanics in which you will basically have to know every single thing about forces in the entire world lol. By this, I mean you will have to keep in mind all the laws of Newton, interaction of things and statics.
I'm not a civil engineer, but I did consider it at one stage last year and this is what I gathered from my research. My suggestion is to do your own lengthy research, go to Open Days and get to know what it entails, then have a feel for it and whether you are interested in it or not. Key thing about choosing engineering is to have an interest in the field you go into.
Also, mechanical and civil engineering are closely tied in their fundamentals at the start, however both lead off to very different paths. Statics and Dynamics are very different things.
Here is a sample plan of the program at UNSW. Do please consider other universities too and pick the one that suits you best.
http://www.engineering.unsw.edu.au/civil-eng
Best of Luck