from review http://www.ams.org.proxy.lib.ohio-state.edu/mathscinet-getitem?mr=1403921 of a survey paper by Paul Seymour, we find...

In 1993, Seymour, Neil Robertson, Daniel Sanders, and Robin Thomas, after trying to read the Appel-Haken proof, decided to supply their own proof, in which the data are available in electronic form, which can be checked by hand or computer. They confirmed that the four-color theorem is true and provable by the approach used by Appel and Haken.

from review http://www.ams.org/mathscinet-getitem?mr=1403921 of a survey paper by Paul Seymour, we find...

In 1993, Seymour, Neil Robertson, Daniel Sanders, and Robin Thomas, after trying to read the Appel-Haken proof, decided to supply their own proof, in which the data are available in electronic form, which can be checked by hand or computer. They confirmed that the four-color theorem is true and provable by the approach used by Appel and Haken.