1) find out if the Barum Polaris 2 and the Barum Polaris 3 are "axle compatible" with each other. Some tires are, some aren't. If they are, then you can treat them like the same tire for these purposes.
2) 5km is not a lot of wear, but it is some. You want each tire to be within 1/4" rolling circumference as mentioned above. This is not the end of the world though, you could get the new tire shaved a little bit to match your other three. It's doable.
2 is only even an issue if the new tire is axle compatible. If it's not, all 4 must go.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD