One commenter from the video disagreed with the "3 and 4 at the same time" answer, and he tried to prove it by using Bernoulli's equation.
Tank #3 will overflow before #4. By applying Bernoulli's equation, we
can actually calculate the water level (head) difference of the two
tanks to be:
h = (Q / A)^2 / (2 * g)
Where Q is the flow rate between the tanks and where A is the cross sectional area of the connector.
Just throwing some real numbers at this equation for people who don't like equations. Say we have a A = 1.6 square inches (about the size of a quarter), and that Q = 20 gallons per minute (about the speed that your bathtub fills at). The water level in tank #3 will be ~3 inches higher than in tank #4. Also, if you account for friction losses, this height difference will actually be even