hi.you can set it by using related field.

Try this example:

class first_table(osv.osv):
    _name = "first.table"
    _columns = {
       'field1' : fields.many2one('res.partner','Field1'),

        'field2' : fields.integer('Field2'),

}

class second_table(osv.osv):
    _name = "second.table"
    _inherit = "first.table"
    _columns = {
       'field3' : fields.many2one('first.table','Field3'),

        'field4' : fields.related('field3','field1', type='many2one', string='Field4',store=True),

}