But in this production, there're only 3 elements on the right hand side, and I don't see what $4 refers to...

Ok, here is more guessing, if we're talking about GRAMPROG  remember stmtseq and

remember: /* NULL */ /* start a full lexical scope */
{ $$ = block_start(TRUE); }
;

stmtseq : /* NULL */
{ $$ = (OP*)NULL; }
| stmtseq fullstmt
{ $$ = op_append_list(OP_LINESEQ, $1, $2);
PL_pad_reset_pending = TRUE;
if ($1 && $2)
PL_hints |= HINT_BLOCK_SCOPE;
}
;
[download]

then surely $$ = op_append_list(OP_LINESEQ, $1, $2); becomes $2, $3, $4, right?

:)