My opinion is, you could still use a circular cross section and put 2 rows of holes in each tube as you plan to do with 4*4 pipe.
Like this:
If you use a pipe about 5 inches in diameter, the holes could be high enough for your application. Or you could try 6 inches. The amount of water to flood the pipes will increase of course, that is a disadvantage. But the setup will approximately require the same amount of space.
I am not saying that this is better than 4*4, but it could do the trick if you can not find 4*4 pipes.