R4's fastethernet is connected to S6 int Fa0/4, so the path goes through
S5 on int Fa0/24

S5#sh run int fa 0/24
Building configuration...

Current configuration : 115 bytes
!
interface FastEthernet0/24
 switchport trunk encapsulation dot1q
 switchport mode trunk
 mls qos trust dscp
end

S5#

R3#sh run | b class
 class-map match-all dscp26
  match  dscp af31
 class-map match-all dscp12
  match  dscp af12
 class-map match-all dscp18
  match  dscp af21


 policy-map Prob22
  class dscp12
   bandwidth percent 15
  class dscp18
   bandwidth percent 15
  class dscp26
   bandwidth percent 50
!
!
!
!
interface Serial1/2
 ip address 180.40.7.1 255.255.255.224
 max-reserved-bandwidth 80
 service-policy output Prob22