Recommendations for 4th monitor options

Take a look at Jeff's review of Monitor Arms then this product.

Can you mount a extension off your desk like this Desk Mount

Also, this is pretty cheap. I've bought a few times from mono price, these cheap/low end monitor brackets, and I've been very impressed. Especially if you go with their $30 ones or so, they have worked well. (disclaimer: no affiliation with monoprice)

IN the monitor escalation wars around my office, some guys have hit 8 or more, setting monitors on your CPU or cardboard boxes have been popular. Also one guy has built a series of shelves from wood to hold his monitors all in the positions he wants.

Quad Vision makes some nice multi monitor stands http://www.quadvision.co.uk/products/multi-monitor-stand.htm

Your question should really be "Do I need a 4th monitor?" Chances are, unless you're a stock trader (where you'd need instant access to data at pretty much all times), you're not properly using the three you already have. nVidia has the nView tools built into their drivers in Windows that allow you to better use your three monitors. Also, you mention the "I need to move my head to see it all" issue; your problems with that (and a very bad crick in your neck) are just going to get worse if you move to a fourth monitor.

Having previously used both 3- and 4-monitor setups, I prefer having three, since a center screen will allow you to "center" on what you need to be productive.

That said, if you already have the fourth monitor, just go for it. There were some good monitor arms mentioned in the other answers that are far less than the cost of a new monitor.