My current tire set up has two different tires in the front and the back. I have Michelin in the front and GoodYear in the back, plus they are different thread. Like AK Rover mentioned, the new spare will not affect our 4 wheel drives. The mechanic you went to is not educated about 4 wheel drive or better yet-- he wanted to sell you the set of tires.

Don't sweat, put on the spare and now just make sure you do a 5 tire rotation.