I just finished First strike and in that novel,Dr Halsey seems to arrive at the conclusion that the star chart in the Sigma Octanus 4 artifact lead to reach, however i remember reading in Fall of Reach that Cortana used the symbols from the same artifact to get the Pillar of Autumn to Installation 04. So which one is it? I'm confused.

Here are the relevant bits from First Strike:

A third file was an analysis of the symbols John had captured from the alien artifact. According to Ackerson’s notes it, too, was a partial star map. Dr. Halsey returned to the stellar chart reference in the Spartans’ files.

No good. This location had seemingly nothing to do with that reference.

The stellar reference in the alien artifact was…she did the math in her head— “I’ll be God damned,” she muttered. She pulled up star charts and NAV records for confirmation, and checked her math one last time. No question: It was the Epsilon Eridani system. Here.