Hey jpye,
It is unlikely that you have gained 4 pounds in 1 week... it's more likely some of that is water weight.
In any case, you need to get back on your maintenance or weight loss plan right now! That means, first of all, get rid of any leftover holiday goodies before they can do you any more harm. I know that's a hard thing to do--but you may have to do it before you eat them!
If you know you'll be craving it, get away from it! Toss it, don't buy more. Get healthy foods instead.
Get back on your exercise/activity program as well! That can make a big difference if you're eating right.
Don't allow 1 week of overeating to turn into 2 weeks just because it's the end of the year!
You have done so well at losing in the past, and you can do it again. You know how! Get with it!
C'mon, you know you can!
Also, you say you weren't prepared for it--and all I can say is, Christmas comes every year! So now, you know that you have to prepare for the holidays.
Good luck! Get back with it! Be strong here!
Jay