What is the IUPAC name for CH3-CH2-CH2-CH3?

The IUPAC name for CH3-CH2-CH2-CH3 is indeed Butane. To determine the name, one must look at the longest continuous carbon chain present in the molecular structure. In this case, there are four carbon atoms connected in a linear fashion.

The naming convention for alkanes follows the rule of using the prefix that corresponds to the number of carbon atoms in the longest chain, combined with the suffix "-ane," which denotes that the compound is an alkane (a saturated hydrocarbon). For four carbon atoms, the prefix "but-" is designated, leading to the name Butane.

The other options do not represent the correct number of carbon atoms for this structure: Propane contains three carbon atoms, Hexane contains six, and Octane contains eight. Therefore, the only accurate label for the four-carbon alkane is Butane.