Puzzle #220: Borders

How many Canadian provinces have exactly four straight lines for their land borders? For the purposes of this puzzle, a straight line is a line that runs on a line of latitude or of longitude and is primarily land. (Rivers and lakes are not considered to violate this, but a salt water coast does.)

Submit your answer to Gene Wirchenko <genew@telus.net>. Your answer should be in the form of a proof. That means to show how your answer must be correct. The deadline is Wednesday, August 23, 2017 at noon Pacific Time. I will post the answer shortly after.