Which one or will both work correctly in a 97 Legacy L?

Either will work fine. I think the only major difference is Dot 4 has a higher boiling point. I always buy the Valvoline syn power stuff since it's usually on sale when I need brake fluid.

While DOT 4 has a higher boiling point than DOT3 when uncontaminated, water more quickly reduces the boiling point of DOT 4. If you're good about changing brake fluid on a regular basis, DOT 4 is probably a better choice. However, if you only change it when there's a brake problem, DOT 3 could make more sense for you.