99% of the world here at AVS are interested in the 16:9 screen size and 16:9 projectors. That's the format HD is in and will be what eventually everyone will receive from cable and off air , DVD's etc. most of the new good technology for home theaters is in the 16:9 native AR as well.
When and if you watch 4:3 on a 16:9 machine you have to ways to go, one stretched out so everyone is short or two with black bars at the sides.
Then there are some of us that actually like 4:3 and enjoy classic old movies in 4:3 and like them big. Also in my case the limiting factor on my screen size was the width of my room. That's also the limiting factor on how close you will like sitting to any screen. There is just so much peripheral vision we have and it becomes annoying to most if the image gets to wide. If you still watch a lot of SD TV and or have a huge collection of old 4:3 DVD's etc or a narrow room you may want to consider 4:3. most of this type projectors you will be told are for business use only and not suitable for watching movies. That is true in some cases and not true in others.
I buy any new DVD's in 16:9 format and view them with black bars top and bottom. They are still as big as if I had a 16:9 projector because room width dictates that in my case.
Below are a couple screen shots of SD and DVD showing my setup.
From SD HBO
From the Discovery ch
From Hulk DVD 16:9
One last comment on 4:3 projectors and being presentation. They are high lumen projectors and can help if you have ambient light issues. And they are pretty cheep mine is selling in the $650 neighborhood now.
There is no right answer when it comes to your question but in each persons case a different answer. Hope this helped a little and I didn't confuse you more.