What is the ruling on an employee working four hours while being paid for eight, due to insufficient work and with the approval of the higher manager?

If the employee's absence from full-time work is with the approval of his direct manager and those above him, given that they have the authority to determine working hours and there are no tasks requiring full-time presence, then there is no harm in that. Sheikh Ibn 'Uthaymeen and Sheikh Ibn Baz have issued fatwas to this effect, provided that the work does not require the employee's presence.