Hello,
I have a network infrastructure as shown in the attached picture.

I'm working with Boson Netsim as I don't have access to the hardware.
The Gateway is a MLS, the two switches on the left are managed and the one on the bottom is unmanaged.
I have two VLANs: 20 and 99.
On the MLS I have the following config:

!
Version 12.3
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Gateway_1
ip cef
!
!
!
!
!
ip subnet-zero
ip routing
spanning-tree extend system-id
!
!
!
!
!
!
!
!
!
interface FastEthernet0/1
 switchport mode trunk
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 20,24,31
!
interface FastEthernet0/2
 switchport mode trunk
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 20,24,31
!
interface FastEthernet0/3
 switchport mode access
 switchport access vlan 99
!
interface FastEthernet0/4
 switchport mode access
 switchport access vlan 99
!
interface FastEthernet0/5
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface FastEthernet0/6
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface FastEthernet0/7
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface FastEthernet0/8
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface FastEthernet0/9
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface FastEthernet0/10
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface FastEthernet0/11
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface FastEthernet0/12
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface GigabitEthernet0/1
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
interface GigabitEthernet0/2
 switchport mode dynamic desirable
 switchport trunk encapsulation dot1q
!
vtp mode Server
vtp domain NULL
!
interface Vlan 1
 no ip address
 no ip route-cache
!
interface Vlan0099
 ip address 10.0.0.1 255.0.0.0
 no ip route-cache
 no shutdown
!
interface Vlan0020
 ip address 172.20.4.111 255.255.0.0
 no ip route-cache
 no shutdown
!
vlan 99 name RM
vlan 20 name HMI_MELTSHOP
!
router rip
 network 10.0.0.0
 network 172.20.0.0
!
!
!
ip classless
no ip http server
!
!
!
!
!
!
!
line con 0
line aux 0
line vty 0 4
!
no scheduler allocate
end


Now, I can ping interface Vlan 99 from PC 4, directly connected to the MLS, but not from PC 3 which is connected through the unmanaged switch.

What am I missing here?
Thanks in advance for your help.

  If pc 3 is on port 3 or 4 then there is no reason it shouldnt ping and is probably a glitch with your emulation program .  Your config is correct.  For an unmanaged switch thats all you need is a access port in vlan 99 .

Yes, I think that the program is the problem. The same config with Cisco Packet Tracer works.
Thank you.