Poly-urethane, is nothing but urethane that has short arm ( of carbons) to make polymer with. This short molecules make polymer as such that they cover an entire surface at one time. Now by applying four coat, you just made that thin layer protective not only along the surface, but vertically upward too.
But, as far as I know you are just covering the surface, there is no chemical bond between wood surface and poly urethane. So if you have a hole somewhere, water can slowly seep over a period of time through and lift that urethane from under the surface of wood. Imagine peeling paint. That's why you need to sand it all if you want to use poly again, because you cannot bond on top of old poly which may have dirt, grease, wine to make coverage through out the surface.
To make this water repellent even stronger for outdoor use, you increase the length of the carbons( i.e. Helmans polyurethane) but because in these polyurethane molecules, chemical bond only happens with urethane- urethane, it is not as strong as regular polyurethane because urethane- urethane bond is farther apart in Helsmann due to long chain of carbons on end.
Second, Red-oak if you used is no no. It is open grain along the length. Water can seep by capillary on this. White oak is closed grain and holds ten times better.
Tung oil also does not make bond, but it does not just sit on the surface, it seeps inside. Some Tung oil ( I have used it and it works great) seeps deep inside those wood pores (and likely water may also only reach that depth when exposed ) stays there and hardens. Now imagine trying to push water through those pore, if there is any because those are covered with oil. Not happening or very unlikely, because water hates oil. So after few years those oil which hardens very slowly or seeps deep inside or get lost, just coat another coat. No sanding needed.
Polyurethane is not inferior at all, in fact it's resist daily abuse better and you can use anywhere where water is not a problem. Plus UV rays also deteriorate those urethane-urethane bond. Just need right place to use, indoor flooring, furniture etc etc.
Your urethane may last a while, but try one coat on piece of wood and soak it in water and see. It will eventually peel.
Second, try another coat of poly on top of one month old poly, it is feel.