Brake fluid vs DOT 3 vs DOT 4 — does it matter?

The one caveat: DOT 4 absorbs moisture slightly faster than DOT 3, so the service interval is technically shorter. In practice this doesn't matter because most people never change their brake fluid anyway (they should every 2-3 years). Use DOT 4, you're good.

DOT 4 is backward compatible with DOT 3 — it has a higher boiling point (446°F dry vs 401°F for DOT 3). You can absolutely use DOT 4 in a DOT 3 system. You cannot go the other direction (DOT 3 in a DOT 4 system). And never use DOT 5 (silicone-based) in any system designed for DOT 3/4 — completely different chemistry. For your Outback, DOT 4 is a fine upgrade.