Four meanings is I. A. Richards's way of describing the different kinds of meaning a poem can carry at once: sense (what is being said), feeling (the emotional effect), tone (the speaker's attitude), and intention (the poem's purpose or aim). The idea helps readers notice that poetic language often works on more than one level at the same time.
Example
The winter road is white and bare;
its silence chills me to the bone.
The line gives sense, feeling, tone,
and hints at why the speaker walks alone.