6400 by rr

6*4*0*0=0
6*4*0+f(0)=1
6-4+0+0=2
6-4+f(0)+0=3
6-4+f(0)+f(0)=4
(6+4)/(f(0)+f(0))=5
6+4*0+0=6
6+f(4*0)+0=7
6+s(4)+0+0=8
6+4-f(0)+0=9
6+4+0+0=10
6+4+f(0)+0=11
6*s(4)+0+0=12
6*s(4)+f(0)+0=13
6*s(4)+f(0)+f(0)=14
f(6)/f(4)/(f(0)+f(0))=15
-6+f(4)-f(0)-f(0)=16
-6+f(4)-f(0)+0=17
-6+f(4)+0+0=18
-6+f(4)+f(0)+0=19
(6+4)*(f(0)+f(0))=20