Stanza 4 Summary

Get out the microscope, because we’re going through this poem line-by-line.

A blue sky out of the Oresteia
Arches above us. O father, all by yourself

Wowzers—it looks like the speaker is finally letting us in on what this extended metaphor is representing.
She addresses the statue as "father" here, so we're going to assume that it symbolizes her dad.
The fact that the statue is ruined (coupled with all the death-talk we've been hearing) makes us think that the speaker's father is dead.
So, it seems like this whole poem is an extended metaphor for the speaker dealing with the loss of her dad and the scars that put on her psyche.
(Biography note time: Sylvia Plath's dad died when she was really young, and a lot of her poems are about dealing with that trauma. Check out "Daddy" for another famous example.)
It seems that by using the image of this fallen giant statue, the speaker is relating how huge the loss of the male presence was in her life.
The speaker kind of carries this daughter-mourning-father thing through with another classical reference, this one to Aeschylus's tragic trilogy, the Oresteia.
It specifically seems to reference the way Electra mourns her murdered father, Agamemnon, in The Libation Bearers, the second play of the trilogy.
(Go to "Symbols, Imagery, Wordplay" for more on Electra and Agamemnon.)
We wonder why the speaker references the "blue sky" of the Oresteia. Could it a reference to the realm of the gods, those forces that have the power to bring death down on humanity whenever they feel like it? Maybe reading on will give us some insights…

You are pithy and historical as the Roman Forum.

The speaker references another ancient structure here with the Roman Forum, which today is a big ole ruin. (Check it out here.)
It seems obvious why the speaker describes the Forum as "historical," but the reason why she describes is as "pithy" seems a little harder to figure out. "Pithy" is usually used to describe a statement that sums something up really succinctly—you know, like without using a lot of words.
Could the speaker be saying that the image of this ruined Forum-Colossus succinctly sums up the way she feels about her dead father?
Let's read on to find out.

I open my lunch on a hill of black cypress.

Whatever it is, she takes a break from her daily caretaker duties and contemplates the ruins on "a hill of black cypress." We wonder why she chooses a hill with this particular kind of tree.
Well, the mention of the color black does put us in kind of a funereal mindset, because it's a color of mourning and all that.
Sounds like not the most fun place to chow down.

Your fluted bones and acanthine hair are littered

The speaker returns to the imagery of the ruined Forum by describing the ruined statue's "fluted bones and acanthine hair."
(Come again? Both fluted and acanthine describe the kind of ancient columns found in the Forum. Check out "Symbols, Imagery, Allegory" for the deets.)
The mention of bones also reminds us of the skeleton of the father who's died.