I have seen a lot of people driving cars with all season tires all the time and don't think is safe. My colleague used to do it up to two years back when he couldn't stop in traffic, was no snow on the road , just a small portion with ice , he locked the wheels , brakes zero , he totaled the car .Not sure if winters would have saved him that time, but now he is using winter tires on cold season. Anyway , my first reply was to use two in the back IF YOU CAN NOT AFFORD FOUR , because the thread is asking for this answer . There are many articles about this, here is another one :
At the and of the day he/she decides what to do , one , two four , just provide the correct answer to the question.