Here’s a secret decoder ring for the part numbers of the 4000 series of the Cisco Integrated Services Routers.
First digit = the family, all are 4
Second digit = the sub-family with 4 (highest performance), 3 (middle performance), and 2 (lowest performance)
The third digit = total number of slots, the sum of NIM and SM
The fourth digit = 1, identifying the first in that series. Allows for incrementing for the subsequent platforms in the series.