Insert Polygon into Postgis

I was trying to insert a polygon (I think it's the same as a bbox but not sure yet) into a postgis table. But I only found scaterred examples where the columns declaration was missing... So here's a concret example.
The SQL:

SELECT AddGeometryColumn( 'my_table', 'geo_bbox', 4326, 'POLYGON', 2);
UPDATE my_table 
 SET geo_bbox=GeomFromText('POLYGON((0 0,4 0,4 4,0 4,0 0))',4326)
 WHERE id='my_pk';

Where '4326' is the standard/normal/usual coordinnates system: longitude/latitude. You can do the following to get more info about '4326':

select * from spatial_ref_sys where SRID=4326;

To check if everything is working you can do:

SELECT ST_Contains(geo_bbox, GeomFromText('POINT(2 2)',4326)) 
    FROM my_table
    WHERE clef='my_pk';

It should return 't' for 'true'.

Errors I've bumped into and their "gotchas":

• parse error - invalid geometry
  

  UPDATE my_table 
   SET geo_bbox=GeomFromText('POLYGON(0 0,4 0,4 4,0 4,0 0)',4326)
   WHERE clef='my_pk';
  ERROR:  parse error - invalid geometry
  CONTEXT:  SQL function "geomfromtext" statement 1
  

  
  You forget the double '((' and '))'. There is a double '((' because Polygon can be describe like this: POLYGON((first_shape), (second_shape))
  
• geometry contains non-closed rings
  

  UPDATE my_table 
   SET geo_bbox=GeomFromText('POLYGON((0 0,4 0,4 4,0 4))',4326)
   WHERE clef='my_pk';
  ERROR:  geometry contains non-closed rings
  CONTEXT:  SQL function "geomfromtext" statement 1
  

  You forget the last coordinates, you need to repeate the first point twice:
  POLYGON((down_left, down_right, up_right, up_left, down_left))
• geometry contains non-closed rings
  

  UPDATE my_table 
   SET geo_bbox=GeomFromText('POLYGON((0 0,4 0,4 4,0 4,0 10))',4326)
   WHERE clef='my_pk';
  ERROR:  geometry contains non-closed rings
  CONTEXT:  SQL function "geomfromtext" statement 1
  

  That's because the finishing point (0,10) is not the same as the starting point (0,0)
• other: invalid geometry
  I also got an other way to get an "invalid geometry" but I don't remember how to reproduce it... postgis does some validation if your coordinates are not valid.

Postgis official documentation: Creating object syntaxe

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