So I'm trying to find the 4th cg in the "???" gallery. Anyone know what to do because I feel lost and I'm replaying the moment to see if I missed it.
Not sure if you already figured it out or not, but here's the guide; every locked CG in the gallery that has 
icon while you are pointing it was only be able to unlock by playing "no romance" route.
To do this, you must not accept any dateable characters as a boyfriend (now Macsen and Zhokhar)
You can using [Trailmakers] to find those CG if you don't have enough time or didn't save the choice part.