Did you check the last page of your previous bill? There may be something at the bottom in small print letting you know of the rate increase. 

It is permitted under the Wireless Code, as long as Koodo notifies you within 30 days before the change.

These are the last messages and invoices I recieved, there's no mention of a prince change.

You need to click the “View PDF” button in order to look through your bill.

Ok, yeah it does say at the end that it will increase. But why is it changed to an additional $4 every month now for no reason?

This is quoted from a Koodo page: 

Price increases are sometimes necessary so that Koodo can deliver better service and coverage. We always consider other alternatives first, such as adjusting costs or increasing efficiency, but sometimes a price increase is the best option for ensuring continued quality service.

It’s time to switch. Many cheaper options. It surprises me how providers get away with this . It’s essentially a scam. 

If you don't need to buy a phone for awhile, check any prepaid plan. You may find better deal. ( And no need worry about sudden price increase)