Since he's just 4 months old, Riley still has a ways to go before he'll be able to sit up by himself. Maybe in a month or so. Standing will come even further down the road, but he still likes to try. The one thing he doesn't have a problem doing, however, is sticking things in his mouth. If it's within his reach, it will go in.
I coaxed him into sitting up a second time, but he wasn't as focused the second time around. I think he was more determined to try to get the camera.