It looks as though the PVC elbow isn't very far into the rubber coupler. The super smooth surface of the elbow probably reduces the grip between it and the coupler. I'm not sure that the sizes are correct either.

I would ditch that setup and buy a 3.5" to 4.0" coupler, a 45* 16 gauge aluminum tube elbow, and a 9" long filter with 4" I.D.