Unless you are planning to run an expedition in the extreme cold, huge snow fall and lots and lots of rain I would just go with basic 3 season. I made a mistake of going for one of the North Face 4 season tents. It was heavy, expensive and pain to set. I went back exchanged it for rugged 3 season (north face nebula) put some work into it and it works fine since 1998.

I think that what you do with the tent and how you prepare it plays more important role than what season rating it has. Sealing seams (even if manufacturer tells you that they were sealed), proper set up and general maintenance will make your tent withstand more than you think.

4 season tent will have a tougher floor, stronger poles, better reinforcement on stitching, will hold temp little better sometimes at the cost of ventilation, will be sturdier and most likely free standing without pegs for quick setup (pegs will be used to get full benefits of the four season).

3 season tents can be "converted" into the above. You can replace your poles with stronger ones or you can just clear the snow when it accumulates. In order to make it sturdier you can just pay more attention when anchoring. Use ground cloth or an in-tent liner for extra ground insulation. As far as temp and wind "chillage" goes I guess you can just get an extra fleece liner for your bag or some extra clothing and better sleeping pad. Obviously if you are in -10 with wind chill to -30 4 season will show its benefits. I stayed out with mine down to 0 and it was nice warm and cozy inside. It was snowing and very windy but inside was toasty.

4 season tent in the summer is little pointless. It gets too hot so you have to remove the rain fly but that means that you are going to get soaked when storm passes by. I had the same experience when I bough a 0 degree bag and thought I was set for life. I very quickly changed my mind during one of the 80 degree summer nights when I woke up shivering all wet from the sweat. I guess it got little too warm.

Matt