This will be the final servicing mission performed by the Space Shuttle.  After this one or the other spacecraft, Hubble or Shuttle, will be decommissioned before a servicing mission 5 will be necessary.

Nasa: Servicing Mission 4 Essentials

What a remarkable piece of engineering the Hubble has been.  One of the true intersections between science and engineering and art and even perhaps spirituality.  If you have looked at the images from Hubble and haven't felt humbled at some point along the way, well, you ain't no geek.

Hubble's triumphs continue to accumulate thanks to a unique design that allows astronauts to repair and upgrade the telescope while it remains in orbit. Repairs keep the telescope functioning smoothly, while upgrades to the instruments bring a slew of new discoveries and science.

Review of the Servicing 4 mission. 

This is fascinating...

At Goddard engineers and astronauts work together developing tools and techniques to resurrect two malfunctioning science instruments currently on orbit inside Hubble.

Of course the astronauts are integrally involved in the training, I keep forgetting they are no longer still rocket jockeys but engineers and physicists.

Detail of what a servicing mission entails.  Cool video from the Space Telescope Operations Control Center (STOCC).

and crew training.