I'm new to the hobby, been flying with 1s tinyhawk.

I just got a Cinelog25, which is supposed to take 4s batteries. Am I able to use 3s 450mah batteries in order to go under 250g? Are there disadvantages to using 3s vs 4s batteries?

For slow cinematic flights, flying a 4S quad on 3S should not make much difference. You may actually get more flight time due to reduced weight of the lipo pack.

I would not advise flying it on a 2S pack because then your motor KV will result in degraded performance.

As an example, I have flown a 6S race spec quad on 4S and it flew great.

CineLog25 is good. But loud. Flight time not great.
The frame and 1404 4500kv motors are heavy for 2.5" props.

3S batteries should be fine. But 2S will have approx 50% of thrust of 4S, and thus struggle to maneuver with the heavy frame.

No real disadvantage of 3S vs 4S. But the smaller battery you use the more it will struggle to deliver amps, and will wear out faster.

FYI my friend has CineLog25 and uses Tattu 4S 650mah.

A possible way to increase flight time is to buy CineLog30 frame, and transfer everything onto it.
The 4500kv would run 3" props on 3S.