I completed this exercise and found there was no STP instance for vlan 146 on SW1. Even if I create an SVI on it and another switch. I wasn't expecting this and can't explain the behaviour. I got the same result when I pasted the solution configs in. STP instance exists on all other switches except SW1. Root is SW4, this is reported on SW2 & SW3 too despite the direct connections. So the BPDUs must be traversing SW1 despite there being no STP instance for it.
SW1(config-if-range)#do show span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0017.e01a.b180
Cost 19
Port 23 (FastEthernet0/21)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0023.0467.6880
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.3 P2p
Fa0/19 Altn BLK 19 128.21 P2p
Fa0/20 Altn BLK 19 128.22 P2p
Fa0/21 Root FWD 19 128.23 P2p
Fa0/22 Altn BLK 19 128.24 P2p
Fa0/23 Altn BLK 19 128.25 P2p
Fa0/24 Altn BLK 19 128.26 P2p
SW1(config-if-range)#do show int tru
Port Mode Encapsulation Status Native vlan
Fa0/19 desirable 802.1q trunking 146
Fa0/20 desirable 802.1q trunking 146
Fa0/21 desirable 802.1q trunking 146
Fa0/22 desirable 802.1q trunking 146
Fa0/23 desirable 802.1q trunking 146
Fa0/24 desirable 802.1q trunking 146
Port Vlans allowed on trunk
Fa0/19 1-4094
Fa0/20 1-4094
Fa0/21 1-4094
Fa0/22 1-4094
Fa0/23 1-4094
Fa0/24 1-4094
Port Vlans allowed and active in management domain
Fa0/19 1,146
Fa0/20 1,146
Fa0/21 1,146
Fa0/22 1,146
Fa0/23 1,146
Fa0/24 1,146
Port Vlans in spanning tree forwarding state and not pruned
Fa0/19 none
Fa0/20 none
Fa0/21 1
Fa0/22 none
Fa0/23 none
Fa0/24 none
SW2(config-if-range)#do show span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0017.e01a.b180
Cost 19
Port 21 (FastEthernet0/19)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0022.be79.0f00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Root FWD 19 128.21 P2p
Fa0/20 Altn BLK 19 128.22 P2p
Fa0/21 Altn BLK 19 128.23 P2p
Fa0/22 Altn BLK 19 128.24 P2p
Fa0/23 Desg FWD 19 128.25 P2p
Fa0/24 Desg FWD 19 128.26 P2p
VLAN0146
Spanning tree enabled protocol ieee
Root ID Priority 32914
Address 0017.e01a.b180
Cost 19
Port 25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32914 (priority 32768 sys-id-ext 146)
Address 0022.be79.0f00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/23 Root FWD 19 128.25 P2p
Fa0/24 Altn BLK 19 128.26 P2p
SW3(config-if-range)#do show span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0017.e01a.b180
Cost 19
Port 25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0021.d819.0e80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Desg FWD 19 128.21 P2p
Fa0/20 Desg FWD 19 128.22 P2p
Fa0/21 Desg FWD 19 128.23 P2p
Fa0/22 Desg FWD 19 128.24 P2p
Fa0/23 Root FWD 19 128.25 P2p
Fa0/24 Altn BLK 19 128.26 P2p
VLAN0146
Spanning tree enabled protocol ieee
Root ID Priority 32914
Address 0017.e01a.b180
Cost 19
Port 21 (FastEthernet0/19)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32914 (priority 32768 sys-id-ext 146)
Address 0021.d819.0e80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Root FWD 19 128.21 P2p
Fa0/20 Altn BLK 19 128.22 P2p
SW4(config-if-range)#do show span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0017.e01a.b180
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0017.e01a.b180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Desg FWD 19 128.21 P2p
Fa0/20 Desg FWD 19 128.22 P2p
Fa0/21 Desg FWD 19 128.23 P2p
Fa0/22 Desg FWD 19 128.24 P2p
Fa0/23 Desg FWD 19 128.25 P2p
Fa0/24 Desg FWD 19 128.26 P2p
VLAN0146
Spanning tree enabled protocol ieee
Root ID Priority 32914
Address 0017.e01a.b180
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32914 (priority 32768 sys-id-ext 146)
Address 0017.e01a.b180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/21 Desg FWD 19 128.23 P2p
Fa0/22 Back BLK 19 128.24 P2p