I am installing a new dimmer for a ceiling track lighting with 5 (6w LED) light bulbs. The old switch box is plastic and only has 2 wires (1 black, 1 red). The new dimmer switch has 4 wires (1 black, 2 red, 1 green). My concern is the ground wire (green wire). How safe is it to just cap it off? I have done some research and it seems I am allowed to just cap it off if my current switch box is plastic and has no ground wire. But this scares me.

I
am allowed to just cap it off if my current switch box is plastic
Yes.

But this scares me.
Why, the switch you are using now is not grounded. What's different?