In the Dark

Lt. Granderson says the passive radar signal is coming from four Luyang III-class destroyers, which is the NATO reporting name for a Chinese People's Liberation Army Navy (PLAN) Type-052D guided missile destroyer, their latest class of guided missile destroyer. The Type-052D is a larger variant of the Type-52C (Luyang II-class), the 52C was built in 2002 to have equivalent capabilities to the US Navy's Arleigh Burke-class, and the new 52D was first built in 2012 and designed to compete with the upgraded capabilities of the new Flight-IIA Technology Insertion and Flight-III Arleigh Burke's. So a 52D Luyang III-class destroyer would be an even match for the Nathan James, making her severely outgunned by four of them.

The doctor on board wears a single gold bar. In the Navy, this is an Ensign. As a Doctor the lowest rank he would have would be a Lieutenant, two silver bars.

Edit: Prior to this season Doc Rios was an enlisted surface force independent duty corpsman with the rank of Chief Petty Officer, they are highly trained medics who serve as the primary healthcare provider on smaller ships like frigates, destroyers and attack submarines and aren't actually doctors, they are just called "Doc" as a nickname and sign of respect for the vital role they play. After the pandemic the military is extremely short on doctors so Rios is attending medical school, someone joining the Medial Corps and starting medical school becomes a commissioned reserve officer with the rank of O-1, which would be an Ensign in the Navy, upon graduating medical school they are promoted to O-3, which would be a Lieutenant. So Rios' rank is correct for this situation, in fact by season five Rios is a Lieutenant indicating he finished medical school and is an actual doctor by that point.