On a standard scale 4. 0 is the highest GPA you can get, however some schools and classes weight their grades meaning that if got a 4. 0 it could actually be higher then that causing you to get a higher then 4. 0 grade.
The highest GPA that you can possibly get is a 4.0. Once you go into college it is possible to get a 5.0. College grading scales are different than high school scales.