There are a number of things you can do if you have more than four hours to spare in Abu Dhabi. Make sure you arrange a transit visa with your airline before you leave and you will be able to explore the city and its surrounds.
Visit Ferrari World the largest indoor theme park, open from 11am to 8pm daily.
Indulge in some retail therapy at the Souk Qaryat Al Bery.
Visit the landmark Sheikh Zayed Grand Mosque which is open daily from 9am to 10pm , except on Friday morning, which is for worshippers only.
If you belong to an airline frequent flyer programme you may be able to take advantage of the facilities at one of the seven airport lounges.