Well I get this question so often. I think if you want a four season room then just build an addition and get it over with. I live in the Northeast and I love my porch, but I really can't use it in the winter. But thru the Spring, Summer and Fall its' just great. Thats where I read my Sunday paper! Where I fall asleep or hang around with my kids.
In late fall, when it really starts getting cold, we close it up for the winter. The three seasons we get out of it each year is something we look forwar to opening is the Spring. Can't you give up that one season!
In my opinion a four season room is just another room added to the home. But the screen porch, oh - the breeze.

Here is a link that might be useful: My Album

I have a 4 season, all glass sunroom off of my kitchen. It looks great, but I'm taking it down to build a new addition. In the summer, it is too hot-a window a/c doesn't cut it, and in the winter it gets very cold, need kerosene heater to supplement. Because it is open to the rest of my house, it makes for heating and cooling very difficult, very inefficient.

I guess I am confused but I though a 4 season porch is just a 3 season plus heat and AC enabling you to use it year round?

In the OP's original message it says no heat or AC so I am not sure how this becomes a four season porch???