Well, first you need to replace the bad brake line and inspect the other lines.
Since it's likely that you haven't flushed the brake system in the past year, I'd suggest you flush the system, install compatible synthetic fluid, and as part of the process flush the brakes at all four wheels.
But to answer your question, the brakes are tied diagonally on your Taurus (LF and RR on one line, LR and RF on the other) so you really should do all four to be safe.
-Rod