Train Cascade>>Opencv Error: Insufficient memroy (Failed to allocate 1569600004 bytes)

My training set has 500 positive images and 1000 negative images. My rig's memory capacity is 7.7GB and of which 31% is normally used. I tried to allocate about 4 GB for the training but it gets error. Strangely it says failed to allocate 1569600004 bytes.