constants.py: raise phosh-flavour image size to 4GB

it's 3023M installed on enchilada

constants.py

@@ -58,7 +58,7 @@ FLAVOURS = {
             'iio-sensor-proxy',
         ],
         'post_cmds': ['systemctl enable phosh'],
-        'size': 3,
+        'size': 4,
     }
 }