Could someone please explain the use of Functions 1-4 in Z3TA+2's mod matrix.

from the manual:
 

As a source:
Bipolar. Value of Function1 to Function4.
Can have positive and negative values.
 
As a destination:
Functions 1 to 4. These work as virtual destinations to allow for
intermediate calculations, and they can be used as source in a
lower row in the matrix.
 

 

I would suggest using the approach made popular by some of the early synth-pioneers in the 70's:

Mess with the knobs to make funny noises until you get one you like.
 
A similar approach is nicely summarised in this exchange from Douglas Adams' Hitchhiker series:
 
TRILLIAN:
Hey, that sounds better! Have you managed to make some sense of the controls?

FORD:
No, we just stopped fiddling with them.
 
Frink.