You need to go through the whole document text instead of words.
Another thing, your Regexp doesn't work: you need to escape dots, and {4,4}
should be {4}
Sub findIDs()
Dim oDoc As Document
Dim obj_RegEx As Object
Dim nextID As String
Dim result As Variant
Dim id As Variant
Set oDoc = ActiveDocument
Set obj_RegEx = CreateObject("VbScript.RegExp")
obj_RegEx.Global = True 'match all occurrences
obj_RegEx.Pattern = "RS\.[0-9]{4}/[0-9]{2}\.[0-9]{5}"
Set result = obj_RegEx.Execute(oDoc.Content.Text)
For Each id In result
Debug.Print "Found: " & id.value
Next
End Sub