Seriously, I'm trying to think of how to recreate the right stiffness and elasticity in a sturdier material than cardboard and plastic. Jack was fussing the other night and I handed him this kleenex box. He has been endlessly fascinated with it since.
See, the problem with the typical "toys" available for four month olds is that even though a four month old can grab and hold onto things, they don't understand that letting them go makes the toy go away. Once they let go, the toy is forgotten. Jack has long attention span to watch things dangling above him, though he's not coordinated enough to hold onto a loose object for a sustained period of time.
But this box? Once he works his hand into it, this box can't get away from him. He can gnaw on it. He can swing it around. He can feel its edges and get his other hand into it. He can stare at the bright, contrasty pattern. He can move it up and down and feel his fist rattle inside: cool cardboard and soft tissue. It holds his interest for a really long time.
Best toy ever.