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 21, HIGHLAND PARK, IL 60035-0021
PO BOX 22, HIGHLAND PARK, IL 60035-0022
PO BOX 23, HIGHLAND PARK, IL 60035-0023
PO BOX 24, HIGHLAND PARK, IL 60035-0024
PO BOX 25, HIGHLAND PARK, IL 60035-0025
PO BOX 26, HIGHLAND PARK, IL 60035-0026
PO BOX 27, HIGHLAND PARK, IL 60035-0027
PO BOX 28, HIGHLAND PARK, IL 60035-0028
PO BOX 29, HIGHLAND PARK, IL 60035-0029
PO BOX 30, HIGHLAND PARK, IL 60035-0030
PO BOX 31, HIGHLAND PARK, IL 60035-0031
PO BOX 32, HIGHLAND PARK, IL 60035-0032
PO BOX 33, HIGHLAND PARK, IL 60035-0033
PO BOX 34, HIGHLAND PARK, IL 60035-0034
PO BOX 35, HIGHLAND PARK, IL 60035-0035
PO BOX 36, HIGHLAND PARK, IL 60035-0036
PO BOX 37, HIGHLAND PARK, IL 60035-0037
PO BOX 38, HIGHLAND PARK, IL 60035-0038
PO BOX 39, HIGHLAND PARK, IL 60035-0039
PO BOX 40, HIGHLAND PARK, IL 60035-0040
PO BOX 41, HIGHLAND PARK, IL 60035-0041
PO BOX 42, HIGHLAND PARK, IL 60035-0042
PO BOX 43, HIGHLAND PARK, IL 60035-0043
PO BOX 44, HIGHLAND PARK, IL 60035-0044
PO BOX 45, HIGHLAND PARK, IL 60035-0045
PO BOX 46, HIGHLAND PARK, IL 60035-0046
PO BOX 47, HIGHLAND PARK, IL 60035-0047
PO BOX 48, HIGHLAND PARK, IL 60035-0048
PO BOX 49, HIGHLAND PARK, IL 60035-0049
PO BOX 50, HIGHLAND PARK, IL 60035-0050
PO BOX 51, HIGHLAND PARK, IL 60035-0051
PO BOX 52, HIGHLAND PARK, IL 60035-0052
PO BOX 53, HIGHLAND PARK, IL 60035-0053
PO BOX 54, HIGHLAND PARK, IL 60035-0054
PO BOX 55, HIGHLAND PARK, IL 60035-0055
PO BOX 56, HIGHLAND PARK, IL 60035-0056
PO BOX 57, HIGHLAND PARK, IL 60035-0057
PO BOX 58, HIGHLAND PARK, IL 60035-0058
PO BOX 59, HIGHLAND PARK, IL 60035-0059
PO BOX 60, HIGHLAND PARK, IL 60035-0060
PO BOX 61, HIGHLAND PARK, IL 60035-0061
PO BOX 62, HIGHLAND PARK, IL 60035-0062
PO BOX 63, HIGHLAND PARK, IL 60035-0063
PO BOX 64, HIGHLAND PARK, IL 60035-0064
PO BOX 65, HIGHLAND PARK, IL 60035-0065
PO BOX 66, HIGHLAND PARK, IL 60035-0066
PO BOX 67, HIGHLAND PARK, IL 60035-0067
PO BOX 68, HIGHLAND PARK, IL 60035-0068
PO BOX 69, HIGHLAND PARK, IL 60035-0069
PO BOX 70, HIGHLAND PARK, IL 60035-0070
PO BOX 71, HIGHLAND PARK, IL 60035-0071
PO BOX 72, HIGHLAND PARK, IL 60035-0072
PO BOX 73, HIGHLAND PARK, IL 60035-0073
PO BOX 74, HIGHLAND PARK, IL 60035-0074
PO BOX 75, HIGHLAND PARK, IL 60035-0075
PO BOX 76, HIGHLAND PARK, IL 60035-0076
PO BOX 77, HIGHLAND PARK, IL 60035-0077
PO BOX 78, HIGHLAND PARK, IL 60035-0078
PO BOX 79, HIGHLAND PARK, IL 60035-0079
PO BOX 80, HIGHLAND PARK, IL 60035-0080
PO BOX 81, HIGHLAND PARK, IL 60035-0081
PO BOX 82, HIGHLAND PARK, IL 60035-0082
PO BOX 83, HIGHLAND PARK, IL 60035-0083
PO BOX 84, HIGHLAND PARK, IL 60035-0084
PO BOX 85, HIGHLAND PARK, IL 60035-0085
PO BOX 86, HIGHLAND PARK, IL 60035-0086
PO BOX 87, HIGHLAND PARK, IL 60035-0087
PO BOX 88, HIGHLAND PARK, IL 60035-0088
PO BOX 89, HIGHLAND PARK, IL 60035-0089
PO BOX 90, HIGHLAND PARK, IL 60035-0090
PO BOX 91, HIGHLAND PARK, IL 60035-0091
PO BOX 92, HIGHLAND PARK, IL 60035-0092
PO BOX 93, HIGHLAND PARK, IL 60035-0093
PO BOX 94, HIGHLAND PARK, IL 60035-0094
PO BOX 95, HIGHLAND PARK, IL 60035-0095
PO BOX 96, HIGHLAND PARK, IL 60035-0096
PO BOX 97, HIGHLAND PARK, IL 60035-0097
PO BOX 98, HIGHLAND PARK, IL 60035-0098
PO BOX 99, HIGHLAND PARK, IL 60035-0099
PO BOX 100, HIGHLAND PARK, IL 60035-0100
PO BOX 101, HIGHLAND PARK, IL 60035-0101
PO BOX 102, HIGHLAND PARK, IL 60035-0102
PO BOX 103, HIGHLAND PARK, IL 60035-0103
PO BOX 104, HIGHLAND PARK, IL 60035-0104
PO BOX 105, HIGHLAND PARK, IL 60035-0105
PO BOX 106, HIGHLAND PARK, IL 60035-0106
PO BOX 107, HIGHLAND PARK, IL 60035-0107
PO BOX 108, HIGHLAND PARK, IL 60035-0108
PO BOX 109, HIGHLAND PARK, IL 60035-0109
PO BOX 110, HIGHLAND PARK, IL 60035-0110
PO BOX 111, HIGHLAND PARK, IL 60035-0111
PO BOX 112, HIGHLAND PARK, IL 60035-0112
PO BOX 113, HIGHLAND PARK, IL 60035-0113
PO BOX 114, HIGHLAND PARK, IL 60035-0114
PO BOX 115, HIGHLAND PARK, IL 60035-0115
PO BOX 116, HIGHLAND PARK, IL 60035-0116
PO BOX 117, HIGHLAND PARK, IL 60035-0117
PO BOX 118, HIGHLAND PARK, IL 60035-0118
PO BOX 119, HIGHLAND PARK, IL 60035-0119
PO BOX 120, HIGHLAND PARK, IL 60035-0120
PO BOX 121, HIGHLAND PARK, IL 60035-0121
PO BOX 122, HIGHLAND PARK, IL 60035-0122
PO BOX 123, HIGHLAND PARK, IL 60035-0123
PO BOX 124, HIGHLAND PARK, IL 60035-0124
PO BOX 125, HIGHLAND PARK, IL 60035-0125
PO BOX 126, HIGHLAND PARK, IL 60035-0126
PO BOX 127, HIGHLAND PARK, IL 60035-0127
PO BOX 128, HIGHLAND PARK, IL 60035-0128
PO BOX 129, HIGHLAND PARK, IL 60035-0129
PO BOX 130, HIGHLAND PARK, IL 60035-0130
PO BOX 131, HIGHLAND PARK, IL 60035-0131
PO BOX 132, HIGHLAND PARK, IL 60035-0132
PO BOX 133, HIGHLAND PARK, IL 60035-0133
PO BOX 134, HIGHLAND PARK, IL 60035-0134
PO BOX 135, HIGHLAND PARK, IL 60035-0135
PO BOX 136, HIGHLAND PARK, IL 60035-0136
PO BOX 137, HIGHLAND PARK, IL 60035-0137
PO BOX 138, HIGHLAND PARK, IL 60035-0138
PO BOX 139, HIGHLAND PARK, IL 60035-0139
PO BOX 140, HIGHLAND PARK, IL 60035-0140
PO BOX 141, HIGHLAND PARK, IL 60035-0141
PO BOX 142, HIGHLAND PARK, IL 60035-0142
PO BOX 143, HIGHLAND PARK, IL 60035-0143
PO BOX 144, HIGHLAND PARK, IL 60035-0144
PO BOX 145, HIGHLAND PARK, IL 60035-0145
PO BOX 146, HIGHLAND PARK, IL 60035-0146
PO BOX 147, HIGHLAND PARK, IL 60035-0147
PO BOX 148, HIGHLAND PARK, IL 60035-0148
PO BOX 149, HIGHLAND PARK, IL 60035-0149
PO BOX 150, HIGHLAND PARK, IL 60035-0150
PO BOX 151, HIGHLAND PARK, IL 60035-0151
PO BOX 152, HIGHLAND PARK, IL 60035-0152
PO BOX 153, HIGHLAND PARK, IL 60035-0153
PO BOX 154, HIGHLAND PARK, IL 60035-0154
PO BOX 155, HIGHLAND PARK, IL 60035-0155
PO BOX 156, HIGHLAND PARK, IL 60035-0156
PO BOX 157, HIGHLAND PARK, IL 60035-0157
PO BOX 158, HIGHLAND PARK, IL 60035-0158
PO BOX 159, HIGHLAND PARK, IL 60035-0159
PO BOX 160, HIGHLAND PARK, IL 60035-0160
PO BOX 161, HIGHLAND PARK, IL 60035-0161
PO BOX 162, HIGHLAND PARK, IL 60035-0162
PO BOX 163, HIGHLAND PARK, IL 60035-0163
PO BOX 164, HIGHLAND PARK, IL 60035-0164
PO BOX 165, HIGHLAND PARK, IL 60035-0165
PO BOX 166, HIGHLAND PARK, IL 60035-0166
PO BOX 167, HIGHLAND PARK, IL 60035-0167
PO BOX 168, HIGHLAND PARK, IL 60035-0168
PO BOX 169, HIGHLAND PARK, IL 60035-0169
PO BOX 170, HIGHLAND PARK, IL 60035-0170
PO BOX 171, HIGHLAND PARK, IL 60035-0171
PO BOX 172, HIGHLAND PARK, IL 60035-0172
PO BOX 173, HIGHLAND PARK, IL 60035-0173
PO BOX 174, HIGHLAND PARK, IL 60035-0174
PO BOX 175, HIGHLAND PARK, IL 60035-0175
PO BOX 176, HIGHLAND PARK, IL 60035-0176