Kirk and the other captain had four silver.
Which brings up another error: Kirk still had four pips, even when he insists being called "Commander" instead of "Captain".
The question is, was he was being demoted in rank or in position? According to the pips his rank didn't change, so addressing him as "Captain" would still be correct.
Which brings up another question: When a Captain in rank serves as a first officer, do you address him as Captain? I'm sure that would cause some confusion on the bridge!