Stanza 4 Summary

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

That in the mortar—you call it a gum?

In this chunk of the poem, she's basically looking around the laboratory, asking about things she sees.
The first thing she notices is a bit of "gum" (this could be any kind of sticky substance) in the poison-maker's "mortar" (basically a little bowl you put things in to grind them up).
We kind of feel bad for the guy making the poison. This sounds like trying to focus on something with your annoying little brother around ("What's that? How about that? Or that?").
On the other hand, these lines help us to see that the lady who's speaking is curious and maybe a little nervous, too.

Ah, the brave tree whence such gold oozings come!

Apparently the "gum" she sees in the mortar comes from a tree. The gold-colored sap oozes out, and you collect it and use it to poison people. How nice.
This lady is obviously really into poison, so she compliments the poison tree by calling it "brave." (In this case that means something like "great" or "excellent.")

And yonder soft phial, the exquisite blue,

Now she points to a bottle (a "phial"—nowadays we might say "vial") with blue stuff inside it.
She calls the blue potion "soft " and "exquisite," which we think is a nice touch. It helps to give this whole laboratory scene a kind of strange beauty, with all these colorful gums and bottles lying around.
Our speaker sounds a little enchanted by the things she sees around her.

Sure to taste sweetly,—is that poison too?

The color of the potion makes her think it would be likely to taste sweet.
She wants to know if it's poison, too.
In case you haven't noticed, our speaker is totally obsessed with poison, unable to think about anything else.