In naphtha cracking process, C4R4 refers to C4 residual obtained after separation of 1,3-butadiene, isobutylene, 1-butene, and cis- or trans-2-butene from C4 raffinate stream which mainly consists of n-butane. Normally C4R4 is a side product in tert-butyl alcohol plant if C4R3 is used for feed.