I have a black, white, green, and bare copper wire in my fixture but only a black,white, and bare copper wire in the junction box. The instructions tht came with the fixture say tht I should put the green wire as well as the copper wire from the fixture together and then attach those to the bare copper wire in the junction box however that is not working. I have also tried attaching the 3 wire combination to the green screw or "GND" screw attached to the junction box but tht has not worked either. What do I do?
In any recently made appliance, green is only legal for one purpose: equipment safety ground. Green wire cannot be re-tagged as a hot or neutral so it is guaranteed to be ground.
Bare wire is also only legal for ground.
They are all grounds. Simply wire them together.
Fixtures sometimes have 2 ground wires if there are 2 parts of the fixture which each should be grounded, e.g. a chain-hung chandelier: a chain is not a valid grounding path, so a ground wire must be in the cord running down the chain. The other wire would ground the rose.
Ground is strictly an emergency current path, and does not flow any current normally. Electricity needs 2 wires, so 2 more wires must be connected: black and white. The fixture would work without any grounds at all, but it wouldn't be safe.