I'm in the market for an Atari 2600 for my son who loves retro game consoles. He wants a 6 switcher, but I've found that those are a little bit more pricey than the four switchers. 

I have found that there is no functional difference between the two. The difference is that the difficulty switches for the left and right controls are placed on the back of the 4 switcher as opposed to on the front panel where the difficulty switches are located on the 6 switcher.

Also, I've noticed that the 4 switch version is usually less expensive on eBay.

en.wikipedia.org/wiki/Atari_2600_hardware