Nejprve k aliasu nepotřebujete AS
. takže to můžete udělat takto:
$this->db->select('
exhibit.*,
S1.*,
S2.*,
S3.*,
S4.*,
')
->from('exhibit')
->join('stand S1', 'exhibit.ex_pref_one = S1.stand_id', 'LEFT')
->join('stand S2', 'exhibit.ex_pref_two = S2.stand_id', 'LEFT')
->join('stand S3', 'exhibit.ex_pref_three = S3.stand_id', 'LEFT')
->join('stand S4', 'exhibit.ex_pref_four = S4.stand_id', 'LEFT')
->where('whatever')