This is a potentialy bad situation. At best you might end up with a long cold night with little sleep and lots of shivering. A four season tent is good to keep the elements of you but I have never seen a tent keep me warm. This is the responsibilty of my sleep system and my clothes. My favorite system is the one used by the US Marine Corps. I've spent many repeated cold nights in the system. It is not light weight, however it handles extream conditions with ease. The coldest ive tried was a -30, with snow(not that the snow mattered at that temp). I winter camped in the Sawtooths south of Burley Idaho with the Boy Scouts. Also I appreciated the removeable liners/bags while on deployment with the Corps (I am forever a hospital Corpsman) in the frozen/baked desert of Kuait.
Bottom line, dont skimp on your sleeping bag, maybe on the tent but the bag is what is going to protect you from the elements. Think of it as your coat while you sleep because if you get a cheep bag you might very easily be wearing your coat while you sleep. Hypothermia happens at warmer temperatures to.