[f:[$0>][@@\f;!\1-]#%]r:   { reduce n stack items using the given basis and binary function }

1 2 3 4  0 4[+]r;!." " { 10 }
1 2 3 4  1 4[*]r;!." " { 24 }
1 2 3 4  0 4[$*+]r;!.  { 30 }