Why is there an exponent 4 after the brackets in sp3?
The electron configuration of carbon is canonically written as 1s22s22p2. What you now can do is invoke orbital hybridization and combine the electrons in the 2s and 2p orbitals. Then you get sp3 hybrid orbitals with a total of 4 electrons in them. The 1s electrons stay where they are. To mark the new hybrid orbitals as separate, you put brackets around them.

You then end up with an electronic configuration of 1s2(2sp3)4.