help on 91 celica o2 sensor

while removeing the engine from my son's car he forgot or missed the o2 sensor between the engine & firewall pulling the wires from the connector.
i'm trying to figure out the right sequence to rewire it.it goes into a 4 wire square type connector. if anyone can help i would appreciate it

??the O2 sensor is on the exhaust manifold near the fan,not the firewall?im confuse,secribe what im looking at, i might be able to help you out.