Plus your tires will actually be circular after having that done.

I wouldn’t get any more tires. I assume they couldn’t patch the tire that went flat.

It may not be noticeable since most roads are crowned and your tires are spinning at slightly different rates normally. If on the rear, it's like taking a long gradual bend on an interstate. If you have a limited slip differential, it could cause problems. why not have the tire diameters measured?

Subaru recommends replacing all. In my opinion, no need to replace all tires if tire profile is similar. You can drive with one new tire. 4,000 miles on the tires is not much. Might cause a little difference in handling at higher speeds but might not be noticeable in town and at lower speeds. Ride around for a few days and see how your car feels.

2/32 inch difference in diameter can cause damage to the transfer case and transmission.

But for 4000 miles, I doubt they wore that much.

If you only have 4000 miles on the tire and tread depth is sufficient (which it should be unless it is way out of alignment or the tires are run with too little or too much air), then there is absolutely no reason to replace your three good tires. In fact, it is waste of your money and natural resources. Many of the tires that are replaced are not properly managed and end up in piles in some rural location (where they occasionally catch fire (worked for an environmental agency for over 30 years and saw this frequently).

Assuming it was a nail or something sharp that punctured the tread, then it is relative easy to fix a tire (sidewall damage is not repairable). You can buy a repair kit for around $5.00 where it only requires that you widen the hole and then insert a plug and pull it out quickly (rubber cement on a rubber strip). It is very quick and painless to do anywhere safe. An inexpensive portable air compressor will do the job to refill the tire too (nice to have in case you have a low tire as well).

You shouldn't need to replace the other 3 tires (at 4k miles) because you replace one.

What happens to those 3 tires should you replace them? They get sold as used tires to someone they didn't BS about matching tires.

AWD has nothing to do with any of this equation.