Plum heads and ringnecks, I find, can take a little bit more work to get them 'hand friendly".
Have you only had him for 4 days? if so, if will take a little while for him to get comfortable with you, as well as his new cage and surroundings. It might take anywhere from 1 week to a month for him to get fully comfortable.
Have you found a treat he really likes yet? If so, put him in his cage and feed him treats through the bars. Eventually, open the cage door and see if he'll take the treats from your hand through the open door. Once he's ok with eating from your hand, move on to the next step.
Try putting him on a stand, and feeding him small treats. Eventually, get him to step up onto your hand, still feeding him treats. If he sits there for a while, great. If he flies off, it's no big deal, try again. Don't push him too hard, if he flies off again, thats the end of the training session for now, try again in 15 minutes. For now, only give him treats for sitting on your hand - don't worry about any other treat training.
Some tips: Make sure that there are no noises or distractions while you're training. No TV, so other family members, stuff like that. Also try to make all your movements slow and obvious.