4L
For life
Example
I'm a Toyota guy 4L; can't beat them for reliability

Related Slang
Gang gang | I'm with you |
BFF | Best friends forever |
BFF4L | Best friends forever for life |
Squad | Group of friends |
Based | Agree |
Tight | Close |
4eva | Forever |
4EAE | Forever and ever |
People use 4L (for life) to succinctly express their love for a person, group, or item. For example, a T Swift fan might post "I'm a swiftie 4L; doesn't matter how mid her next album is" to express their undying devotion to Taylor.
On social media, you might encounter 4L in users' usernames, when they've named their account after a thing they love. A user whose username is AvacadoToast4L, for example, is likely really into avocado toast.