The Compressed Red Lightsaber Crystal is on Level 2, Raxus
Prime. After the "Landing Area" and "Thruster Interior" sections of
Raxus Prime you will enter a tunnel. When you reach the exit of the
tunnel you will see a crashed starship. (NOTE: Holocron#3, which
just gives Force Points, is immediately on the left here). Holocron
#4, containing the Compressed Red Lightsaber Crystal, is hidden
behind what looks like a round wall of the starship near the cliff.
You can see this wall near the 11 O'clock position when you're
exiting the tunnel. To watch a video of someone busting through
that wall and using the saber crystal visit the "Related links"
section below or visit
forceunleashed.org/gameplay/holocron-locations/ for a list of all
Holocrons.