1. Monitor 1 LG Mini DisplayPort to VGA (active) (you already have this)
2. Monitor 2 Viewsonic V3D231 Mini DisplayPort to DVI/HDMI passive adapter
3. Monitor 3 Viewsonic V3D231 Mini DisplayPort to DVI/HDMI passive adapter
4. Monitor 4 Viewsonic V3D231 Mini DisplayPort to DVI/HDMI single link active adapter
I saw that you have 3D capable displays on the Viewsonic, if you want to use that feature it will mean to increase the bandwidth. In this case from step 2 to 4 you will require Active Dual Link DVI or Active HDMI adapters.
I suggested to use on step 4 a single link active adapter as the display does not exceed 1080p, they are cheaper to buy than the dual link.
I hope this helps.