A PO BOX number can be less than or more than four digit. For post office boxes, the general rule is that each box has its own ZIP+4 code.
The last four digit is often one of the following:
1. Zero plus the last three digits of the box number (eg. PO BOX 1001, Los Angeles CA 90001-0001).
2. The last four digits of the box number (eg. PO BOX 9991, Twin Peaks CA 92391-9991).
3. If the box number is less than four digits, enough zeros prepended to the front of the box number to produce a four-digit number(eg. PO BOX 1, Shreveport LA 71161-0001).
However, there is no uniform rule, the zip code plus four is for reference only, the ZIP+4 code must be looked up individually for each box in the USPS offical website.
PO BOX 421518, SAN FRANCISCO, CA 94142-1518
PO BOX 421519, SAN FRANCISCO, CA 94142-1519
PO BOX 421520, SAN FRANCISCO, CA 94142-1520
PO BOX 421521, SAN FRANCISCO, CA 94142-1521
PO BOX 421522, SAN FRANCISCO, CA 94142-1522
PO BOX 421523, SAN FRANCISCO, CA 94142-1523
PO BOX 421524, SAN FRANCISCO, CA 94142-1524
PO BOX 421525, SAN FRANCISCO, CA 94142-1525
PO BOX 421526, SAN FRANCISCO, CA 94142-1526
PO BOX 421527, SAN FRANCISCO, CA 94142-1527
PO BOX 421528, SAN FRANCISCO, CA 94142-1528
PO BOX 421529, SAN FRANCISCO, CA 94142-1529
PO BOX 421530, SAN FRANCISCO, CA 94142-1530
PO BOX 421531, SAN FRANCISCO, CA 94142-1531
PO BOX 421532, SAN FRANCISCO, CA 94142-1532
PO BOX 421533, SAN FRANCISCO, CA 94142-1533
PO BOX 421534, SAN FRANCISCO, CA 94142-1534
PO BOX 421535, SAN FRANCISCO, CA 94142-1535
PO BOX 421536, SAN FRANCISCO, CA 94142-1536
PO BOX 421537, SAN FRANCISCO, CA 94142-1537
PO BOX 421538, SAN FRANCISCO, CA 94142-1538
PO BOX 421539, SAN FRANCISCO, CA 94142-1539
PO BOX 421540, SAN FRANCISCO, CA 94142-1540
PO BOX 421541, SAN FRANCISCO, CA 94142-1541
PO BOX 421542, SAN FRANCISCO, CA 94142-1542
PO BOX 421543, SAN FRANCISCO, CA 94142-1543
PO BOX 421544, SAN FRANCISCO, CA 94142-1544
PO BOX 421545, SAN FRANCISCO, CA 94142-1545
PO BOX 421546, SAN FRANCISCO, CA 94142-1546
PO BOX 421547, SAN FRANCISCO, CA 94142-1547
PO BOX 421548, SAN FRANCISCO, CA 94142-1548
PO BOX 421549, SAN FRANCISCO, CA 94142-1549
PO BOX 421550, SAN FRANCISCO, CA 94142-1550
PO BOX 421551, SAN FRANCISCO, CA 94142-1551
PO BOX 421552, SAN FRANCISCO, CA 94142-1552
PO BOX 421553, SAN FRANCISCO, CA 94142-1553
PO BOX 421554, SAN FRANCISCO, CA 94142-1554
PO BOX 421555, SAN FRANCISCO, CA 94142-1555
PO BOX 421556, SAN FRANCISCO, CA 94142-1556
PO BOX 421557, SAN FRANCISCO, CA 94142-1557
PO BOX 421558, SAN FRANCISCO, CA 94142-1558
PO BOX 421559, SAN FRANCISCO, CA 94142-1559
PO BOX 421560, SAN FRANCISCO, CA 94142-1560
PO BOX 421561, SAN FRANCISCO, CA 94142-1561
PO BOX 421562, SAN FRANCISCO, CA 94142-1562
PO BOX 421563, SAN FRANCISCO, CA 94142-1563
PO BOX 421564, SAN FRANCISCO, CA 94142-1564
PO BOX 421565, SAN FRANCISCO, CA 94142-1565
PO BOX 421566, SAN FRANCISCO, CA 94142-1566
PO BOX 421567, SAN FRANCISCO, CA 94142-1567
PO BOX 421568, SAN FRANCISCO, CA 94142-1568
PO BOX 421569, SAN FRANCISCO, CA 94142-1569
PO BOX 421570, SAN FRANCISCO, CA 94142-1570
PO BOX 421571, SAN FRANCISCO, CA 94142-1571
PO BOX 421572, SAN FRANCISCO, CA 94142-1572
PO BOX 421573, SAN FRANCISCO, CA 94142-1573
PO BOX 421574, SAN FRANCISCO, CA 94142-1574
PO BOX 421575, SAN FRANCISCO, CA 94142-1575
PO BOX 421576, SAN FRANCISCO, CA 94142-1576
PO BOX 421577, SAN FRANCISCO, CA 94142-1577
PO BOX 421578, SAN FRANCISCO, CA 94142-1578
PO BOX 421579, SAN FRANCISCO, CA 94142-1579
PO BOX 421580, SAN FRANCISCO, CA 94142-1580
PO BOX 421581, SAN FRANCISCO, CA 94142-1581
PO BOX 421582, SAN FRANCISCO, CA 94142-1582
PO BOX 421583, SAN FRANCISCO, CA 94142-1583
PO BOX 421584, SAN FRANCISCO, CA 94142-1584
PO BOX 421585, SAN FRANCISCO, CA 94142-1585
PO BOX 421586, SAN FRANCISCO, CA 94142-1586
PO BOX 421587, SAN FRANCISCO, CA 94142-1587
PO BOX 421588, SAN FRANCISCO, CA 94142-1588
PO BOX 421589, SAN FRANCISCO, CA 94142-1589
PO BOX 421590, SAN FRANCISCO, CA 94142-1590
PO BOX 421591, SAN FRANCISCO, CA 94142-1591
PO BOX 421592, SAN FRANCISCO, CA 94142-1592
PO BOX 421593, SAN FRANCISCO, CA 94142-1593
PO BOX 421594, SAN FRANCISCO, CA 94142-1594
PO BOX 421595, SAN FRANCISCO, CA 94142-1595
PO BOX 421596, SAN FRANCISCO, CA 94142-1596
PO BOX 421597, SAN FRANCISCO, CA 94142-1597
PO BOX 421598, SAN FRANCISCO, CA 94142-1598
PO BOX 421599, SAN FRANCISCO, CA 94142-1599
PO BOX 421600, SAN FRANCISCO, CA 94142-1600
PO BOX 421601, SAN FRANCISCO, CA 94142-1601
PO BOX 421602, SAN FRANCISCO, CA 94142-1602
PO BOX 421603, SAN FRANCISCO, CA 94142-1603
PO BOX 421604, SAN FRANCISCO, CA 94142-1604
PO BOX 421605, SAN FRANCISCO, CA 94142-1605
PO BOX 421606, SAN FRANCISCO, CA 94142-1606
PO BOX 421607, SAN FRANCISCO, CA 94142-1607
PO BOX 421608, SAN FRANCISCO, CA 94142-1608
PO BOX 421609, SAN FRANCISCO, CA 94142-1609
PO BOX 421610, SAN FRANCISCO, CA 94142-1610
PO BOX 421611, SAN FRANCISCO, CA 94142-1611
PO BOX 421612, SAN FRANCISCO, CA 94142-1612
PO BOX 421613, SAN FRANCISCO, CA 94142-1613
PO BOX 421614, SAN FRANCISCO, CA 94142-1614
PO BOX 421615, SAN FRANCISCO, CA 94142-1615
PO BOX 421616, SAN FRANCISCO, CA 94142-1616
PO BOX 421617, SAN FRANCISCO, CA 94142-1617
PO BOX 421618, SAN FRANCISCO, CA 94142-1618
PO BOX 421619, SAN FRANCISCO, CA 94142-1619
PO BOX 421620, SAN FRANCISCO, CA 94142-1620
PO BOX 421621, SAN FRANCISCO, CA 94142-1621
PO BOX 421622, SAN FRANCISCO, CA 94142-1622
PO BOX 421623, SAN FRANCISCO, CA 94142-1623
PO BOX 421624, SAN FRANCISCO, CA 94142-1624
PO BOX 421625, SAN FRANCISCO, CA 94142-1625
PO BOX 421626, SAN FRANCISCO, CA 94142-1626
PO BOX 421627, SAN FRANCISCO, CA 94142-1627
PO BOX 421628, SAN FRANCISCO, CA 94142-1628
PO BOX 421629, SAN FRANCISCO, CA 94142-1629
PO BOX 421630, SAN FRANCISCO, CA 94142-1630
PO BOX 421631, SAN FRANCISCO, CA 94142-1631
PO BOX 421632, SAN FRANCISCO, CA 94142-1632
PO BOX 421633, SAN FRANCISCO, CA 94142-1633
PO BOX 421634, SAN FRANCISCO, CA 94142-1634
PO BOX 421635, SAN FRANCISCO, CA 94142-1635
PO BOX 421636, SAN FRANCISCO, CA 94142-1636
PO BOX 421637, SAN FRANCISCO, CA 94142-1637
PO BOX 421638, SAN FRANCISCO, CA 94142-1638
PO BOX 421639, SAN FRANCISCO, CA 94142-1639
PO BOX 421640, SAN FRANCISCO, CA 94142-1640
PO BOX 421641, SAN FRANCISCO, CA 94142-1641
PO BOX 421642, SAN FRANCISCO, CA 94142-1642
PO BOX 421643, SAN FRANCISCO, CA 94142-1643
PO BOX 421644, SAN FRANCISCO, CA 94142-1644
PO BOX 421645, SAN FRANCISCO, CA 94142-1645
PO BOX 421646, SAN FRANCISCO, CA 94142-1646
PO BOX 421647, SAN FRANCISCO, CA 94142-1647
PO BOX 421648, SAN FRANCISCO, CA 94142-1648
PO BOX 421649, SAN FRANCISCO, CA 94142-1649
PO BOX 421650, SAN FRANCISCO, CA 94142-1650
PO BOX 421651, SAN FRANCISCO, CA 94142-1651
PO BOX 421652, SAN FRANCISCO, CA 94142-1652
PO BOX 421653, SAN FRANCISCO, CA 94142-1653
PO BOX 421654, SAN FRANCISCO, CA 94142-1654
PO BOX 421655, SAN FRANCISCO, CA 94142-1655
PO BOX 421656, SAN FRANCISCO, CA 94142-1656
PO BOX 421657, SAN FRANCISCO, CA 94142-1657
PO BOX 421658, SAN FRANCISCO, CA 94142-1658
PO BOX 421659, SAN FRANCISCO, CA 94142-1659
PO BOX 421660, SAN FRANCISCO, CA 94142-1660
PO BOX 421661, SAN FRANCISCO, CA 94142-1661
PO BOX 421662, SAN FRANCISCO, CA 94142-1662
PO BOX 421663, SAN FRANCISCO, CA 94142-1663