A minute amount of grass? Will it show up on a drug-screening?

I had a small amount, maybe 10 puffs of weed at a concert on Sept. 9th. I think I got a new job – I’ll know for sure in a couple of days. Problem is: they screen for drugs. I need to take the job asap, but should I wait a couple of more days before I take it, i.e., make up some sort of prior obligation that I must attend to? Is it a hard-fast rule that marijuana stays in your system for four weeks, as I’ve heard? Should I do a cleanse? I drink tons of water a day, and have been exercising – will that help my odds? Ugggh – the dreaded drug-test. I don’t know what to do! Am I driving myself crazy over nothing?