Just play the game normally and listen to the phone calls especially when you saw a black car coming and the moment it stopped the phone rings.

For this ending, no matter who are those people, just let all of them going inside the building (except the ghost) and enjoy the ending :3

Just politely decline everyone no matter who they are and you will see.

I think this ending is RNG based. At one point, when you start the game at the main menu, you will saw the image down below. (I got it after getting all the 3 main endings)
The developer of security booth has said this ending will be triggered after you completed all 3 main endings. After getting all 3 of the endings, play it again and you will trigger this one. Thanks Kyle :D

If you see this that means you are now triggering Ending 4 and as you are inside the office you will see everything is different from the normal playthrough. You will also see the computer in front of you
showing a game called Rite Of Passage. (cool name not gonna lie.)
Just interact with the computer and start the game. Then just continue proceed the game.
i wont spoil anything for ya. play it yourself,explore a bit and enjoy the creepy ending ;)

For me I done it by just open the gate once and just left it open for the rest of the game. (Don't close the gate because i think it will trigger Ending 2 even when you open the gate back. )

Edit: I might be wrong about this. So I think the possible thing you need to do for this ending is just let the ghost in. After that, you will trigger ending 5.

Just stand in front of the gate and let the car hit you and commit insurance fraud.

At some point when you open and go into the gate on your left when facing the computer. This option will appear.
Go inside and you will get da Ultimate Super Secret ending :D also a Secret Ending achievement for finding it.

I want to thank Meeka Nomicon for finding the endings and make it into a video and share it to all of us.
if you still confused or stuck on what to do, Please check the video she made. I will put the video down below.
And..thanks for viewing my guide i first ever made and also thank you Kyle for making this game. Really liked it. Enjoy the game :)