What size is his tank?
I have a 1 gallon tank for my betta. I did a full water change yesterday because I went out and bought him a plant. You're not supposed to do this because it can put your buddy into shock. I've always done full water changes because I didn't know until recently that your not supposed to, so my betta is used to them. Since it's only been 4 days, you should take your fish out with a net (I don't have a net, so I use a cup instead (if you're going to do this, make sure you wash the cup really well in hot water with NO soap). Once he's out put him in a container (which is why I use the cup), and put some of the old water in it. Then take out about half of the water in the tank. Then fill it tank up again with tap water and then use water conditioner. You're supposed to let the tank sit for a day or so before putting your betta back in, but some water conditioners say it is ok to immediately put your fish back in. I usually let the tank sit for about an hour. Then slowly drain the water out of the cup that your betta is in, and slowly place your betta back in his tank.
That's how you do a successful water change. If you have a really small tank, then I would suggest doing a 100% water change. (I've been doing them for a long time, and my betta has never had any problems)