Not everything has to be a special number. 4 is picked because the ones in charge thought it was a good number, a fair amount of visits, but not too much. Plus, considering how the related forum e-peen and domination rewards are at four (and multiples of four), right now it’s going to be more random to change it to five instead of keeping it at four.

Of course, I’m not speaking on behalf of those in charge. It could very well change in the future.