Will weed from 3 weeks ago show up on a drug test after I smoked 4 days after?

I took the blame for a friend and got arrested for half a gram of weed so i'm being tested before diversion. I smoked 3-4 weeks ago once and smoked 4 days before the test itself..i know I will test positive but is there any way for them to be able to tell i smoked three weeks ago specifically?