I live in VA, do most of my winter backpacking in WV. I have to pray for snow.......this year looks like a dry one, though I haven't checked recently to see what snow cover they may have. My situation seems similar to yours, living in a temperate region more or less. I have a 15 degree bag that I find works great for me even down to single digit temps, with a liner. My tent is an REI Morph (no longer made) - a convertiblea 3/4 season. I think REI gave up on convertibles and I imagine for good reason. Agree with above posters, what makes a winter tent is stoutness, which means more weight than I want to carry in summer. The Morph comes with a liner that zips in to cover all the mesh in the tent, which serves to block wind and improve temperature inside, thereby, and an extra ridge pole for added stability. It has a good vestibule, and one of my criteria for a winter setup is a good vestibule, makes a nice kitchen and wet and muddy clothes repository. The convertible components actually work well, and when left out the tent is reasonably lightweight. As with many REI tents, though, there were issues with poles snapping (having to do with not fully inserting pole ends into attachment points, and it's easy to not seat the pole ends fully).
During most fair weather situations I use my tent in freestanding mode, but in winter (besides the liner, the only difference....) I stake out the sides of the tent, all the stake-out points on the tent, and I use snow stakes (when in snow) to ensure the tent remains stout in a good wind. In summer I don't care too much if a wind gets the better of me, I don't mind getting up at night and staking out the tent.....in 5 degree weather and howling wind that becomes a different matter.
In your climate, I think the 3 season tent is fine. Agree also with other posters, rent some equipment to see for yourself what differences there are. Personal experience is #1 in my book.
Not sure I completely agree with other posters on the temperature thing, though. Many 3 season tents, like mine (Morph and an REI half dome) have a lot of mesh in the main canopy. Because of the air flow through that, a fly does not offer the protection of a solid wall tent. Solid walls will be warmer, don't rely on a fly to function as an inside wall.