Find points that lie within a set of coordinates

SELECT * FROM myTable WHERE 
ST_Within(the_geom, GeometryFromText ('POLYGON((75 20,80 30,90 22,85 10,75 20))', 4326))

<-- replace coordinates as necessary

Sounds like you want the BBOX (Bounding Box) from your points - so ST_Extent would be favourable.

BBOX2D

http://www.bostongis.com/postgis_extent_expand_buffer_distance.snippet

would provide the http://postgis.net/docs/ST_Extent.htm page but the server is having issues