The B-4000 is also known as the WSG-5000 and is available from a number of other vendors, but I hit a dead end tracking down who the actual OEM is for the product. What I do know is that it uses Global Sun GL242202-0TA PC card radio (Figure 1) that's based on TI's ACX100 chipset. This means that it actually can support 22Mbps "802.11b+" operation, although ZyXEL doesn't seem to be making that a selling point in their product literature.
Figure 1: The B-4000's radio
(click on the image for a full-sized view)
The B-4000's design is current-generation since it's powered by the Conexant CX82100 single-chip ARM940T-based processor that's at the heart of many current inexpensive routers. It uses a Kendin KS8721 for the 10/100 WAN Ethernet and KS8995 for the four switched LAN 10/100 ports. Flash, RAM, an SMsC I/O controller and Cirrus Logic PLD make up the rest of the main board.