What kind of problem have you set up using parfor.. a general fact about the Parallel Computing Toolbox is that the parallel tasks have to be pretty intensive/expensive in order for parallelization be effective..
If your computation is trivial then you might not get the speedup you expect.. Not a lot more can be said without looking at what code you have parallelized.
Also, is your machine quad core or a two core hyperthreaded which would show up as 4 virtual cores?
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