Hello, and welcome to the HUBB community.
Air Canada's proposed $4,000 charge to ship your bike Toronto - Argentina is not unreasonable, but understanding why the cost is so high (compared to the cost of air freighting a moto Canada to Europe) requires a bit of an understanding of the air freight marketplace.
A motorcycle is very lightweight relative to the space it occupies in an aircraft. If an air carrier is operating on a relatively short route such as Canada to Europe, and they are not filling the belly with freight - as is the case during the summer on the TransAtlantic routes - then they have an underweight aircraft with empty cubic space available, and any money they can get for filling that space is a windfall. This is why you see offers to ship motos TransAtlantic for about $1,000 in the summer.
On the other hand, if an aircraft is operating on very long, thin routes such as Canada to South America, it is likely departing at close to maximum takeoff weight, and it is probably filled to capacity (either cubic, weight, or both) with airfreight. A 600 pound motorcycle in a standard ULD (uniform load device, otherwise known as a 'can' or air freight container) takes up the same amount of space as 6,000 pounds of packages and boxes that could be put in that same container.
Hence, the air carrier will only be interested in carrying the motorcycle if it can get the same revenue from the moto as it can get from smaller, individual air freight packages that would take up the same cubic space as the motorcycle does.
You might be able to find a better price by calling various air freight forwarders in the Toronto area (you can start your search for them here:
Canadian Air Freight Forwarders) and asking them to quote you a price. These forwarders often buy bulk space from air carriers, and if they can't fill that space, they might offer a good price to you just to cut their losses.
When you call, let them the exact weight of your moto (from your owner manual, don't forget to add the weight of anything in the saddlebags), and when you would like to ship it.
All that notwithstanding, I doubt if you will be able to get a price less than (at the very lowest) $2,500.
Michael