If there was only one Jesus, why don't we just have one story about Jesus' life that has the truth, the whole truth, and nothing but the truth?
One reason is because they have different perspectives. When there is a big event happening, it's interesting to hear about it from different perspectives. The local, St. Louis news will cover a story differently than the national news, or a foreign news agency like BBC. It is not that one of those stories is telling the truth and the others are lying. Rather, they are all targeting different audiences, explaining things in a way that is helpful to different groups of people.
One great way to see the differences in the four Gospels is to see how they all treat the Christmas story - the story of when Jesus was born. It might surprise you that not all of them have all the parts that you remember.
John is the most symbolic Gospel. There is more symbolism and poetic features in his Gospel. The other three, sometimes call the synoptic Gospels because of their similarity, follow a more consistent pattern, but John blazes his trail with seven big miracles that echo the history of God's people, and many confusing statements like Jesus saying "I am the bread of life." John was writing in response to the Gnostics - a first century religion/philosophy that valued spiritual things above the physical. John's one-verse Christmas story would have been surprising to them - that the Word took on Flesh! God became a person. He moved into our community.
Mark is a very fast paced Gospel with lots of action. He had a message to get out to people about who Jesus was, and he wanted people to get it soon! The Christmas story might have been delightful, but there are more crucial things he needed to get to.
Luke emphasizes the outcasts. It is often said that Luke wrote for the Gentiles, and he probably did as the only non-Jewish author in the Bible. But he also wrote for the poor, the sinners, and the women (women were considered to have a lower status than men). While Matthew's story and genealogy connect to Joseph, Luke's Christmas story is from Mary's perspective. Luke doesn't have the gospel message proclaimed to the important wise men from the east, but rather to lowly shepherds.
Matthew was written to a Jewish audience. He spends more time showing how Jesus was the fulfillment of the Old Testament and the promises that God had made about a Messiah, a Savior.
God's blessings to you as you see how God's message of love to you is unique and yet the same in each of the Gospels!