Your original post didn't mention anything about track changes. To work with those, you'd need to use a macro that, instead of replacing the found string, inserts the brackets before & after:
Code:
Sub Demo()
Application.ScreenUpdating = False
With ActiveDocument.Range
With .Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = "<[0-9]{4}>"
.Replacement.Text = ""
.Forward = True
.Wrap = wdFindStop
.Format = False
.MatchWildcards = True
.Execute
End With
Do While .Find.Found
.InsertBefore "("
.InsertAfter ")"
.Collapse wdCollapseEnd
.Find.Execute
Loop
End With
Application.ScreenUpdating = True
End Sub