My invisible 4th child

Many things have changed since having a third child. One of them is that I now have an invisible 4th child. And it's not the kind you'd think. It's not the one that ate all the marshmallows after she was told to stay out of them, or hits her sister with a barbie doll, or shoved 4 DVD's into the DVD player all at once. (I think my kids are still too young to blame it on someone invisible. They just say, 'not me!') This one is a phantom anxiety that bursts forth from the back of my mind occasionally. I feel like one of my kids is 'too' quite and must be up to something, so I start to look around only to notice that all 3 of my kids are in the room with me and are simply playing quitely. I also hear her (I know it's a girl, cause that's what I have) crying somewhere in the house and when I stop and focus, I realize that....nope. Everyone's in the room with me and they're fine. Sometimes I even try to heard my I4C into the minivan after visits to the library, etc... only to look around and remember, 'oh, yeah, it's just the 3'. Very weird. Or I'm just going crazy.
ps-our computer broke about 6 weeks ago and Travis forbade me to go buy one without him because he wanted to pick it out. So I waited patiently for what seemed like an eternity!!! (because all of you who know my husband, know that he plans more for himself to do everyday then anyone could ever possibly accomplish, so he would never have time to go to the store with me and pick out a computer!!) He was off work all of last week and it was still the last day of his vacation before I could get him to go with me on threat of 'if you don't go with me today, I'll go by myself tomorrow and pick it out without you!!' that he finally did it. So we're back online! Hoo-ray!!