As with most issues that arise with a car, it is always a good idea to read the Owner's Manual. The Subaru manual definitely speaks to this issue, and it will give you specifics regarding the acceptable difference in circumference between tires.
What you will find is that, if the 3 remaining tires are fairly new, you can probably buy just one tire. But, the circumference figures in the manual should be used for reference when checking the tires just to be sure. Failure to follow the advice in the manual will lead to damage to your center differential, and that would not be covered by warranty if you damage it by mounting differently sized tires.
Hopefully you will luck out, and your 3 remaining tires will fall within the acceptable specifications. Otherwise, you could have the tread "shaved" on one new tire in order to comply with the size difference. If your tire dealer is not able to perform tread shaving, then you need to find a dealer who will do this, as it would save you from buying 3 additional tires.
Even if you have never read the Owner's Manual previously, this would be an excellent time to start to learn about your car from this valuable reference source.