I wouldn't re-certify it at all. After all the horror stories I've heard about bungled repacking jobs at various re-certification stations, I would rather have a four year old factory vacuum packed unit than a re-certified one. Years ago I was having a beer with a tech from a very well known re-pack place in FL and he assured me he would rather have an original pack, up to eight or nine years old, rather than have one repacked. He went on to say that he had seen several rafts go out the door just as they had come in - basically nothing done but a new certification sticker...and allowed as how these were probably the lucky ones.