There doesn't seem to be an answer to this issue, which looks like an overly aggressive version-identification bug to me.
I tried to suggest a synonym of 'texttemplate' for 't4', and received the message "Version specific synonyms can only be created by moderators".
However, 't4' isn't a version specification, rather its a contraction.
I looked at Version Specific Tag Validation on Tag Synonym Page but there is no answer and it is specifically talking about numeric prefixes, whereas this is a case of a numeric postfix.
Given these are only suggestions that require voting anyway, this logic seems overly aggressive.
Is there a way to override it for non-moderators?