Why would I need 4:3 Ratio projection screen?
If you are using a standard resolution projector (ie 1024x768 is actually 4:3) and your content is primarily "video format" you should buy this aspect ratio screen. If your computer is also putting out the video format (screen resolution is NOT Widescreen) then it is clear that all your information is based on the 4:3 format and this is what to buy. If your laptop is widescreen then you can go either way as you will not have the perfect match if you have a 4:3 ratio projector.
Practical considerations in an installed situation are that all projectors will do 4:3 or 16:9 - however a 4:3 ratio projector will do 16:9. It does this by sqeezing the picture and leaving a white areas at the top and bottom of the viewing area when it changes the aspect ratio to 16:9 (widescreen). However a 4:3 ratio projector run in widescreen to fill a 16:9 screen will not work in 4:3 as it will make the picture taller than the screen white area, unless you move the projector forward.
Most corporate situations use 4:3 at the present time. Most home situations use 16:9 screens.