Do I need a visa for a 4 hour layover in Canada?

In June I am traveling to Spain through Air Canada from the U.S and I have a layover in Toronto both going to Spain, and as well as coming back home. I have searched time and time again for whether I need any kind of Canadian travel visa even just for a 4 hour layover. Different sites give me different answers. Some say no, some say yes, some say if I’m there for less than 48 hours then no and some say no matter how long I stay in Canada I will need a “transit visa.” Can someone give me a clear definite answer? And if I do, how do I acquire a Canadian transit visa as a US citizen with a US passport? Any information would be greatly appreciated.