Do I need a Visa?Fr ICN to Guangzhou to DPS, then DPS to Guangzhou to ICNwith 1 to 4 hrs layover.

Will be traveling to Bali, Indonesia from ICN --> Guangzhou (1 hr layover) --> DPS. Return flight is the same, DPS ---> Guangzhou (but 4 hrs layover) ---> ICN. We are US citizens but reside in S Korea. Do we need a visa during those layover in Guangzhou? Please advise. Thanks.