Scikit-Learn- How to add an 'unclassified' category?
In the supervised learning approach as it is, you cannot add extra category.
Therefore I would use some heuristics. Try to predict probability for each category. Then, if all 4 or at least 3 probabilities are approximately equal, you can say that the sample is "unknown". 
For this approach LinearSVC or other type of Support Vector Classifier is bad
suited, because it does not naturally gives you probabilities. Another classifier (Logistic Regression, Bayes, Trees, Forests) would be better
I am using Scikit-Learn to classify texts (in my case tweets) using LinearSVC. Is there a way to classify texts as unclassified when they are a poor fit with any of the categories defined in the training set? For example if I have categories for sport, politics and cinema and attempt to predict the classification on a tweet about computing it should remain unclassified.