It all depends on the distance you want to view the image from.

A billboard(20'x60') might look completely fine with an 10MP image, that is as low as 5dpi.

I personally have printed canvas prints from images as low as about 70dpi without issues. They look great. Before I send them off though, I use software to resize them to the desired output. Most if not all labs that create canvas prints will also have similar software to do this, but I prefer to do it on my own. I use Perfect Resize from OnOne Software.

To answer your specific question, will you be able to create a 60"x20" canvas from a 4759x1798 image? I think you will just barely be able to squeak by. It isn't going to be "art studio" quality necessarily or be able to be scrutinized very closely, but that isn't typically the purpose of a canvas print.

As noted by other answers, you will have to crop the image on the short end to achieve that aspect ratio though. Since you are already starting with a low resolution image, you may want to alter the aspect ratio of the canvas to get every last pixel that you have out of the image rather then cropping further.

I have seen people turn old Facebook photos into canvas prints at 16"x20". Is the quality good by my standards, no. But they are completely fine with it as we all have different levels of acceptable quality. Also, they aren't viewing it from up close, so it really isn't a problem.