The 3- and 4-digits radars follows the same rules as the others radars: the number MUST read the SAME backward as well as forward.
Examples: 3-digits radars: 3244423, 1123211, 1213121 and so on...there is 4320 such radars per 10 million notes (a full prefix run)
4-digits radars: 4632364, 1234321 (special case called "ladder" note), 9521259, 8620268 and so on. These radars are probably the least obvious to find. It takes a careful look to find them. There is 5040 such radars in a full run of a given prefix.