EMB FLC HAD LESS THAN LEGAL SEPARATION IN ATL CLASS B AIRSPACE.

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AFTER DEP FROM ATL ON RWY 8R; WE CLBED TO OUR INITIAL ALT OF 4000 FT AND TURNED TO OUR ASSIGNED HDG OF 330 DEGS. WHEN WE SWITCHED FROM TWR TO DEP; HE GAVE US ONE MORE TURN IN THAT SAME GENERAL DIRECTION; BUT DID NOT GIVE US A HIGHER ALT AS THEY USUALLY DO (USUALLY 10000 FT). IT WAS VFR AND THE CTLR WAS VERY BUSY. I STATED TO THE CAPT WE WERE GOING STRAIGHT FOR PDK; WHICH WAS AT OUR 12 O'CLOCK POS AND ABOUT 5 MI. WE WERE STILL AT 4000 FT AND COULDN'T GET THROUGH TO DEP CTL TO ASK FOR HIGHER DUE TO FREQ CONGESTION. WE RECEIVED AN AURAL TCASII ALERT AND SPOTTED A CITATION CLBING N OUT OF PDK. IT WAS 800 FT BELOW US AND CLBING AND TRAVELING L TO R TO OUR FRONT. WE ESTIMATED IT AT 1 MI OR LESS IN DISTANCE. WE TURNED L 20 DEGS AND CLBED TO 200 FT ABOVE OUR 4000 FT ASSIGNED ALT FOR COLLISION AVOIDANCE. WE FINALLY GOT THROUGH TO ATC AND ASKED HIM ABOUT THE TFC AND IF IT WAS IFR. HE SAID THEY WERE CHKING INTO IT. SHORTLY THEREAFTER; WE RECEIVED A CLB. I BELIEVE ATC FORGOT TO CLB US; OR ATC AT PDK MISTAKENLY TURNED AND CLBED THE CITATION TOWARDS US ON HIS TKOF. EITHER HE SHOULD NOT HAVE BEEN CLRED FOR TKOF IF THEY WANTED US AT 4000 FT; OR OUR ATC SHOULD HAVE CLBED US WAY BEFORE THEY DID.