Quattro is not that sensitive, if you have one tire with a couple thousand miles on one side and a brand new one on the other, it won't notice or care. The difference of the overall diameter of the tires will be so close, it will be insignificant. People run staggered wheel setups with different tire sizes, but they match them where the difference is so close it doesn't have any bad effect.
If it was me and I didn't care too much, I would just replace the one or else the one pair if the opposite tire had significant wear on it. But if you want to go with a different brand you might as well replace all 4.
But I would ask myself, "what kind of driving do I do with my A4?" If you just commute around and don't do much spirited driving really, I would stick with all-seasons for the fact that they will last longer and save you money. Quattro will hook you up with tons of traction, but you only really need "summer" tires if you're going to be ripping around corners, launching, etc.
It's your call though, I'm between both sides so I don't have a strong recommendation, sorry
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