get_the_author_meta('ID') will return the ID of the author for the current post.
That means this line should work.
get_avatar( get_the_author_meta( 'ID' ), 120);
This will not work
get_avatar( get_the_author_meta( '4' ), 120);
Because you are trying to get a property named '4' inside the user object and that property doesn't exist.
If you wanted to get the avatar for user 4 you would use.
get_avatar( 4, 120);
Because get_avatar takes either a
$email as the first parameter and the
$size as the second.
On a final note the WordPress documentation uses, so check for a formatting error in your html.
get_avatar( get_the_author_meta( 'ID' ), 32 );