If I been doing p90x for 4 weeks then I stop for 1 week should I start over again or just pick up where I left off
No, there is no need to start over just pick up where you left off. You will be fine with the results, it will be the same as if you never stopped.
I my idea, you should continue course where you stopped. a good blog for you http://www.yourfitnessreviews.com/px90-reviews-does-p90x-work