Hello and thank you for the misinformation, Mr Anonymous.
I have the model 3C myself and the filter fits perfectly in it. The dimensions are exactly the same, as you can see from the specifications.

does it fit into the Xiaomi Mi Air Purifier 4?

The size of the filter is not the same - this one fits the Xiaomi Air Purifier pro H - the 3 C is lower, also its filter is slightly shorter/lower.