I do believe that a 4-season tent would be overkill for what you are looking for. By going with a 4-season tent, you will be adding a lot of extra weight, and more likely a longer setup time. It will also be anywhere from $100-1000 more than a tent you will buy from target or walmart. Most 4-season tents on the market are made for the fourth season, winter. They are built storng and use top quality materials to withstand the elements. Most are pre water sealed and have very little ventilation to offer in the summer months.
The 3-season tent, is good for everyting else. If your not going to climb the alps or test your tent out in 4 foot of snow, then a 3-season is the way to go for a good up-grade to you current tent. the first think I noticed about my tent, was the difference in the material of the floor. the coleman that i used to have looked as if it was made from a tarp. a 3-season also has better resistance to waterproofing. But i would always use a seam sealer on top of that for added protection.
It sounds as if you are happy/content with your current tent. Will a 4-season tent keep in the heat a little better? im sure it will.. but not enough to justify the extra $$$. As other people have pointed out. You will be better off spending your hard earned money on quality sleeping pads like therma-rest, ($100) and a sleeping bag made with 700+ goose down material(200-500+), rated at 40 or lower. I dont know what cold is to you, but we all take it differently, some warm up nice and tosty in a 40 deg bag, others need a 0 or -10 deg bag to get the same feeling. If I were you, i would buy a good quality bag for winter/cooler months use, and use your current one for sumer/warmer weather use.
They also make some very nice longjohns now in days. youll pay some where between $30-$50 per top and bottom, but they do make a difference, and are much warmer than the standard cotton ones.
So if you add that all up youll spend about 300-600 per person to keep much warmer than if you bought a new 3 or 4 season tent
I own the Moss Little Dipper 4-season tent. It will be the last tent I own untill there is nothing left of it. Its the only one i have found that works in any weather or season, hot or cold. retailed at 1200+ I picked it up for 400 new. Only downside to it. She weighs in at 12 lbs+. It doesent keep me much warmer in the winter, but it will stand up to 3+ feet of snow, take 30+ mph winds, and wont take on water as long as the level is below the zipper line. Here is a link if you want to see more.
http://www.backcountry-equipment.com/tents/moss_littledipper.html