The fourth symbol is 4 Calling Birds

Result of my research:
“… All these years you’ve been singing “calling birds” when the reality is, it’s colly birds. So what are these birds? A colly bird is a European black bird. Colly means black. It’s my view that it’s the word “coal + ie” said with an little accent? Therefore, a dark, black bird looked like black coal and was called a “coalie” or “Colly Bird.

So, the song is really saying… four crows.  back in medival times this bird was considered a delicacy. In the children’s song “Sing a Song of Sixpence” there are 24 blackbirds backed in a pie. Pies seem to be a real gourmet food in the peak of The Twelfth Night days and were often a sign of status and competition among the wealthy. Dining during this time was a form of entertainment, with food presentations having fireworks, and surprises coming out of them.  …” Source

According to another source, ‘Calling Birds’ means “Either the Four Gospels or the Four Evangelists.” source

In Christian tradition, the Four Evangelists are Matthew, Mark, Luke, and John, the authors attributed with the creation of the four Gospel accounts in the New Testament that bear the following titles: Gospel according to Matthew; Gospel according to Mark; Gospel according to Luke and Gospel according to John.

Matthew – a former tax collector who was called by Jesus to be one of the Twelve Apostles,
Mark – a follower of Peter and so an “apostolic man,”
Luke – a doctor who wrote what is now the book of Luke to Theophilus. Also known to have written the book of Acts (or Acts of the Apostles) and to have been a close friend of Paul of Tarsus,
John – a disciple of Jesus and the youngest of his Twelve Apostles.
They are called evangelists, a word meaning “people who proclaim good news,” because their books aim to tell the “good news” (“gospel”) of Jesus.

The song goes like this:
“On the fourth day of Christmas
my true love sent to me:
4 Calling Birds”

In my reading ‘my true love’ is Jesus Christ who gives me his teaching through the writings of the four Evangelists. Beyond the teaching that they conveyed, they also represent certain qualities that the seeker of liberated life may embody.

Matthew the Evangelist is symbolized by a man who speaks a lot of the Kingdom of Heaven. It reminds us of our human and mortal nature.

Mark the Evangelist is symbolized by a winged lion, a figure of labourous worker and servant. It reminds us to be strong and courageous and to never cease to focus on God.

Luke the Evangelist is symbolized by a winged ox a man that represents man to God. It shows us our Godlike nature, the essence of our true self.

John the Evangelist is symbolized by an eagle, a figure of the sky that remind us to become like God loving, caring and merciful.