If it was my DS - also 4 next week - I would sit him down when he was calm and talk about what it is about pine cones that upsets him. Try and get him to explain how they make him feel, don't push him, but try and let him find his own words.
It is probably some very illogical (to an adult mind) connection that he has made.
As an example, I have another ds who is 2.5. He "sees" things - eg animal faces in the knots on a plank of wood. He had been pulling the wall paper off his wall beside his bed (another story ) last week, and just after bed time he became hysterical because he "didn't like the rabbit". It turned out the rabbit was a shape he could see in the torn out wall paper - so I then had to pull a whole load more off and convince him it had gone home. He has since been telling me on a regular basis that the rabbit has gone home to tea ! I have also spent some hours stripping off the rest of the wallpaper ready for redecorating.