Figure 1 provides a peek inside the 4300. The identity of the 10/100/1000 switch / LAN PHY chip is hidden under the heatsink at the center of the photo, but D-Link told me it's a Broadcom BCM5385. Most of the other important components are tucked under the mini-PCI radio that's soldered into its connector clips. An interesting discovery is that the 4300 actually has two antennas - the external, removable 5dBi dipole you know about and the internal dipole affixed to the board at the lower left corner of the picture.
Figure 1: Inside view of the DGL-4300
(click image to enlarge)
The heart of the 4300 is a Ubicom IP3023 Wireless Network processor, which is an 8-way multi-threaded 32 bit processor clocked at 250MHz. I've seen Ubicom 2000-series devices used before in wireless bridges - the Linksys WET11 and WET54G among others - but this is my first sighting of a 3000-series device.
The 4300's FCCID tells me that the mini-PCI radio is made by Alpha Networks and the photo in Figure 2 taken from the FCC ID documents shows it's based on the AR5213 baseband / MAC, which pegs it as an AR5004X chipset.