Tires: Spare as full time Tire, cold climate, tire industry

Question
I need to replace my all-terrain OE tires on my 2006 Tacoma.  I have a full size spare with no tread wear; 4+ yrs old (I'm assuming).  What is your recommendation on buying three new tires (same model as spare) and utilizing the current spare as the fouth tire?  I now live in West Texas but truck has been in Southeast for previous two years.  I'd make the best of the three replaced tires the spare.

Thanks

Answer
Dan,

Recent bulletins from the tire industry indicate that tires degrade simply due to time.  The age of a tire is important even if the tire is unused.  There some disagreement over how to best express this age limitation, but my take is:

If you live in a hot climate (AZ, CA, NV, TX, and FL) then the limit is six years.  If you live in a cold climate (MN, ND, WI, MT, etc), then the limit is 10 years.  States in between are  ..... ah ........ in between.

so you currently live in TX, but the vehicle spent 2 years in the SE.  so if i do a bit of interpolation, the tires are good for 7 years - and you've got at least 4 1/2 years on them - so you have 2 1/2 years left.

Personally, I don't like this situation and i think you'd be further ahead purchasing 4 tires that are better suited for what you want to do, and leave the spare there - for an emergency.