Tetrameter is a line of verse made up of four metrical feet. The feet can be iambs, trochees, anapests, dactyls, or another regular pattern, depending on the poem. Poets use tetrameter to create a line that feels shorter and quicker than pentameter, while still allowing enough room for rhythm and development.
Example
On SILVER tides the NORTH wind LIFTS the SAIL
And BRIGHT stars SHIVER OVER DARKENED PAILS
Anapestic tetrameter:
"And the sheen of their spears was like stars on the sea" (Byron, "The Destruction of Sennacherib")
Iambic tetrameter:
"Because I could not stop for Death" (Emily Dickinson)
Trochaic tetrameter:
"Peter, Peter, pumpkin-eater" (English nursery rhyme)
Dactylic tetrameter:
Picture your self in a boat on a river with [...] (The Beatles, "Lucy in the Sky with Diamonds")