! some stack cleanup (but all untested)
: (n>z) ( n -- z )
    [ 10 ] dip -10 * 0 1 4array ;

: prod ( z n -- z' ) (n>z) swap comp ;

: pidigit-array ( n -- array ) [ unit 1 ] dip [ pi-stream ] { } make ;

: lifts ( k -- z ) 
    dup 2 * 1+     ! k 2k+1
    [ 2 * 0 ] keep ! k 4k+2 0 2k+1
    4array ;

! for other words, try to split to smaller words, keep constants seperated from the word's functionality, and don't put two implementations in one word. split it and use its name.

! also try to use the cleave and spread combinators.
for example:
z1 first4 { [ :> q ] [ :> r ] [ :> s ] [ :> T ] } spread
although the let solution is good also.

note: all of the above code is untested