"Spring is one of the four seasons."

Translation: Printempo estas unu el la kvar sezonoj.

[Snugglesworth]

When to use El and De still eludes me.

[vikungen]

Think of el as out of in all senses out of can be used, here spring is one out of four possible seasons, thus el.

Other uses could be, if you take something out of a chest el, compare with to take something from a chest de.

[drownloader]

I can't tell if I should give this answer a lingot or downvote it