It is normal if she is bending and moving around on her own. It sounds like she is close to a well baby visit at 4 months so you can ask the doc then just to give you peace of mind. My daughter would sometimes stiffen herself when we picked her up. The doc would check her legs at her visits and was not at all concerned about having to "force" her legs to move around to check range of motion. My husband swore I must have dropped her and she is afraid of being picked up. Now she is fine. Also she would stiffen her legs whenever I would try to bend them myself, like when changing her diaper and trying to get it just right so we didn't end up with back door leaks as we call them. And stiffening their legs when you stand them up is totally normal. I think as parents we are so cautious about our kids and it can be great but we get scared so easily, especially after hearing about certain conditions that are out there. But again, if she is moving fine on her own but appears stiff only at the times youmention, totally normal.