Don't worry it's not another tyre advice thread... Even though title says that

Bought 2 new Vredestein ultrac sessanta tyres around 2K mikes ago, came out last week to find a huge nail sticking out the sidewall rendering it unrepairable! Now I'm looking at buying another set but my Q is, I'm gonna replace the rear tyres to, so I have a full Vredestein set. Should I just buy 4 and keep the part used one as a spare OR should I buy 3 and move the used tyre to the rear!? Seems simple but looking for advise, would prefer to save £120 If poss! Also would 3 new & 1 part used tyre have any affects on having a 4 wheel geo setup (which I'm having done after new bushed ams WALK)
