Having mended many iphone 4 phones I have never had this problem until I had a problem with my own unit. Like you I'm very careful with how I fix things and followed the normal procedure.
When everything was back together the camera worked fine until I put it on to front facing which jams the camera front and back (unless you use the message camera) this is how I take pictures at the moment.
I then ordered a NEW front and rear camera and replaced these and still have the same problem.
If I reset the phone General> reset< reset ALL settings then when it has finished the camera works again UNTIL I press front facing camera then it goes off again and you can see the camera/video button flicker.
So its not software and its not hardware. I would say a break in the soilder on the board is the only other answer to this issue.