When I stayed in Camp 4 a couple of years ago, there wasn't anyone manning the check-in kiosk when I arrived at night. There was a clipboard with a list of all of the sites in the camp. "Check in" meant finding a site on the list that had space (less than 6 people in the site already) and signing one's name on said list. I paid for the site the next morning when someone was working the kiosk.
So unless something has changed, you can arrive and check in whenever you like, provided there is space for you. The sites are shared, so be prepared to make some new friends in your site.
Camp 4 is located in the valley at the same place as the trailhead for upper Yosemie fall. It's on Northside Drive across the street from Yosemite Lodge. There is a parking lot for Camp 4, but keep in mind that you won't be able to drive up to your campsite, so be prepared to carry your equipment and supplies from the car to your site.