Currently the pod instance that a record is created on is encoded in the 4 character of the ID, immediately after the key prefix. There are some more details about the Pod identifier in the question What are Salesforce ID's composed of?
As per Has Salesforce hit an architectural limit?, there are currently 61 pods listed on trust.salesforce.com.
What happens when the 62 possible values allowed by base-62 encoding is exceeded?
Will it move to the fifth character as well?