The fourth shell can hold up to 32 electrons. This is because the fourth shell has four subshells, which are labeled 4s, 4p, 4d, and 4f. The 4s subshell can hold up to 2 electrons, the 4p subshell can hold up to 6 electrons, the 4d subshell can hold up to 10 electrons, and the 4f subshell can hold up to 14 electrons.