Jesus speaks of having already brought the Father glory by completing the tasks He had laid out for Him. He is using the past tense as if everything has already been done, although the cross still lay before Him. How can He say this? Note that Jesus was always obedient to the Father (4:34). By going to the cross Jesus would show all men the extent of God’s love.

John 4:34  “My food,” said Jesus, “is to do the will of him who sent me and to finish his work.

On a personal note, we also bring God glory on earth when we do the tasks He has laid out for us. However, doing things for God that He has not laid out for us does not necessarily bring Him glory. Doing those things is like a small child picking a handful of weeds for her mother. The only beauty they bring to the house is to the mother herself who sees the heart of the child and her motives. God loves our efforts to please Him, but only the things He tells us to do will have any impact on earth.