Officers rushed to the scene of an incident outside the town's Star Hotel after being told that four men were fighting.
One man is said to have been seriously assaulted during the disturbance.
A spokesman for Greenock CID said: "CS spray had to be discharged. It was quite a violent incident.
"Violence of any description will not be tolerated over the festive season, or at any time."
It is understood that CCTV cameras captured the incident, which is said to have started around 9.20pm.
All four accused - aged 20, 21 and two of 24 - were arrested and charged with fighting, police assault, resisting arrest and breach of the peace.
They are due to appear at Greenock Sheriff Court later today.
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