What is difference between PVST and MST?
In PVST mode the bridge priority is configured individually for every VLAN. In MST mode the bridge priority is set for the MST instance, which can include multiple VLANs, and any change to the bridge priority of an MST instance should make all the VLANs that are part of the MST group inherit the same attribute(s).
Here's a small capture from my lab switch that I hope helps.
SW1-3550(config)#spanning-tree vlan ?
WORD vlan range, example: 1,3-5,7,9-11
SW1-3550(config)#spanning-tree vlan 10 priority 32768
SW1-3550(config)#spanning-tree mst ?
WORD MST instance range, example: 0-3,5,7-9
configuration Enter MST configuration submode
forward-time Set the forward delay for the spanning tree
hello-time Set the hello interval for the spanning tree
max-age Set the max age interval for the spanning tree
max-hops Set the max hops value for the spanning tree
SW1-3550(config)#spanning-tree mst 1 ?
priority Set the bridge priority for the spanning tree
root Configure switch as root
SW1-3550(config)#spanning-tree mst 1 priority ?
<0-61440> bridge priority in increments of 4096
SW1-3550(config)#spanning-tree mst 1 priority 32768
You would change the bridge priority for the MST instance and not vlan4 in MST setup. Hence, it would affect all 3 vlans in this case.