Which one will get filled first?

No man. 3rd. As the fourth will start getting water when the 3rd is filled to the top.

@samdx008: I see the flaw in my thought process, I was only thinking of a weak flow rate.

For lower flow rates, 3 and 4 at same time because the water level will be equalized between those 2 containers, assuming both containers tops are at the same height and exposed to the same atmospheric pressure. The top of container 3 is clearly above the bottom side of the connecting tube(3-4) on container 4 so your statement is incorrect.

For faster flow rates, container 1 since it won't the connected tube won't be able to drain enough fluid fast enough.

@Volfield: yes man you are right. But the diagram is drawn with rough hands not to pinpoint accuracy. And flow is weak not to introduce turbulence. 🙂🙂

Pythagorean cup is one of the coolest ways to learn about fluid dynamics and how air pressure affects these kind of systems and a really cool party trick :)

@Volfield: I studied fluid dynamics during my graduation in mechanical engineering.