Small note, Aizen actually had nothing to do with Ulquiorra being an arrancar, Ulquiorra and Stark for that matter are natural arrancar. Doesn't change any points tho
I don’t really know why this is a debate tbh, it was made extraordinarily obvious by the storytelling that Ulquiorra was supposed to secretly be a league above the rest of the Espada.
The Kanji for "4" or "Shi" roughly symbolizes "Death", as "shi" is pronounced almost identically to "shin", which definition is literally "death". The same is also said for Mandarin Chinese as well.
With how Ulquiorra acts around Ichigo, it sounds like he wears the number 4 on his chest like a badge, almost as if he chose the number itself just to mock his enemies. It would fit in with how he appears to look weak, only to whip out the 2nd release form just to flex on Ichigo.
Another thing to keep in mind though, when Yami goes 2nd release his 0 remains. However, when Ulquiorra goes 2nd release his 4 disappears completely and has no number on him. This could just be that Aizen couldn't place a permanent number on him OR it could mean that Ulquiorra IS the strongest Espada and couldn't be ranked. I don't know, that's kind of my thought on this.