A speaker tells someone, flatly, that they won't believe "I love you" no matter how often they hear it. The reason is simple: too many people have said those words without meaning them, and the speaker has been let down enough times that the phrase has lost its weight.
The tone is guarded and tired rather than angry. The final line, "now i'm numb to the 4 letter word," is the heart of it. Numbness isn't dramatic. It suggests someone who once felt something when they heard those words but has stopped reacting as a way of protecting themselves. The title, "4 letters," points to the word "love," shrinking it down to a count of characters, which makes it feel small and empty.
The short, plain lines and lowercase style add to the mood. Nothing is decorated, and that bareness makes the speaker's distrust feel honest. There's a quiet sadness underneath: the speaker is shutting the door on something they may still want.