Hello. Because AlexNet was trained to classify images into 1000 different categories, it has a fully connected layer that will produce an output array of 1000 probabilities. This layer will need to be changed to only produce 4 outputs.
Here is a quick example of how to get the layers of AlexNet and change one layer:
net = alexnet
layers = net.layers
layers(23) = fullyConnectedLayer(4)
After this, you'll want to retrain the network on your new dataset. For a more detailed explanation and code example, check out this video
