How to edit attribute tables using Python/ArcPy?

This should do it and is a little simpler than the examples in the online help for UpdateCursor which is nevertheless worth a read.

I've assumed your shapefile is in a folder called C:\temp and that structuretype is an integer field. If it is a text field just use "3" and "4" instead of 3 and 4.

import arcpy

features = arcpy.UpdateCursor(r"C:\temp\Structures.shp")
for feature in features:
    if feature.structuretype == 3:
        feature.structuretype = 4
        features.updateRow(feature)
del feature,features

Warning: The way I am referencing field values here only works for old style cursors and not for the superior and faster cursors in the Data Access (arcpy.da) module, which was introduced with ArcGIS 10.1.