How to connect 4 monitors to Mac mini 2011
An old thread, but in case anyone finds this via Google, I will be a drive-by good samaritan: I'm running 4x 1920-res monitors with a 2011 mini, as the poster wishes.

Number one is connected via HDMI; number two is connected via Thunderbolt. These two are fast enough for movies and 3D gaming. Number three and number four are connected with two USB adapters, specifically this one: HIS Multi-View II Video Adapter HMV2-MAC-PC USB to DVI Interface (link: http://www.newegg.com/Product/Product.aspx?Item=N82E16815136004 ).

Many of the other brand USB adapters won't do 1920 (check carefully) or, as someone above said, will only do a single display from OS X, but I can confirm that one (at least) works with OS X (Mountain Lion, though I also used this setup with Lion).