These warp arrival coordinates will be the death of me. But thanks for your patience. Likely unnecessary bug-fix.
To find the L4 Tunic, you need the bulk of the Relics, the ladder, and either the L3 or L4 Boomerang or the Mirror Shield, all for the purposes of defeating Mirror-robes. On the bottom floor of Excavation, fully explore it and collect all of the keys so you can go straight north to the upper-most room of the bottom floor of Excavation. You'll have to walk through a lot of walls in really weird ways to get the keys to the locked doors.
Here's a map of the floor, as well as where the walkable walls are. The L4 boomerang is in the southwest corner, and there's a Heart Container in the southeast corner.
Spoiler
To find the L4 Sword, it's a bit of a hike.
Spoiler
Eventually I'll stop being lazy and turn that whole area into something far more impressive, but who has the time for that.
Also, there is a full list of Relics and where to find them in the first post of this thread. It's at the bottom of that thread, three spoiler-boxes from the bottom.
That's a beautiful photo, thanks