Updates a single named field on a part category record identified by `partCategoryId`. 
Use this function when only one column needs to change (for example, renaming a category or toggling its enabled flag) 
instead of passing a full `partCategoryInfo` dictionary to `updatePartCategory`. 
The `field` parameter must be one of the valid column names for the part category table, and `value` is supplied as its string representation.

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### Parameters
**partCategoryId** `Integer`: Id of part category to update<br />
**field** `String`: Valid options are categoryName, parentPartCategoryId, sortOrder, enabled<br />
**value** `String`: Value to update to<br />
**userId** `Integer`:Id of the user executing the function<br />

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### Returns
**# of records modified** `Integer`<br />
**message** `String` None if success<br />

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### Example
```python
# Bump the sort order of the "Electrical" part category
partCategoryId = 15
fieldName = 'sortOrder'
newValue = '4'
userId = system.kanoa.security.getIDPUserId({'userName': 'SYSTEM'})

recordsModified, message = system.kanoa.inventory.updatePartCategoryField(partCategoryId, fieldName, newValue, userId)
print recordsModified, message
```