I’m not sure of the Oxygen permeability of buckets over the course of 3-4 days, but I’d wager the active yeast makes quick work of the minuscule amount introduced via that pathway.
In short, that should not be an issue with 3-4 days worth of vigorous primary fermentation.
Aha. Not an issue, I don’t think, during fermentation. The yeast are actively scrubbing the oxygen. The concerns with the permeability of fermenters that I am familiar with are more along the lines of bulk aging or letting the beer sit for a length of time after fermentation (like a couple weeks or whatever, not specifically aging).
I am not a low-oxygen guru, however. Maybe stainless is more better?