From DD-WRT Wiki
 Hardware Specs
|CPU Speed||480 MHz|
|Flash ROM||8 MB|
|Radios||2.4 GHz and 5 GHz|
|WLAN Max Speed||300 mbps on 2.4Ghz and 450mbps on 5Ghz|
|Switch||4x GigE + 1 GigE WAN|
|USB||1x USB 2.0|
 Flashing Instructions
Its always good practice to read the Peacock Thread for reference on Broadcom Based Routers.
You'll need to download 2 files before you begin this procedure. For those upgrading from an older build, you need not to use the initial trailed mini .chk file (the .chk file with the router name in it is the trailed build for initial flashing only), but you must use the nv64k dd-wrt builds to upgrade with or you'll end up with a bricked router.
And this one for a more feature filled build (Will give you USB function amongst other features)
For going from stock Netgear firmware to DD-WRT on a WNDR4000, installation goes like this:
- Set a static IP such as: 192.168.1.8 subnet:255.255.255.0 gateway:192.168.1.1 on the PC you will be using for the flash.
- Connect the PC to one of the router's LAN ports (unplug your ISP modem for the moment)
- Perform a Hard Reset on the router.
- In your web browser, go to http://192.168.1.1
- Click on “Router Upgrade” in the menu at left.
- Upload the special .chk firmware file, that you downloaded from the link above.
- Wait 5 minutes after the router says it is done, then perform another 30-30-30 reset. Make sure you clear your browsers cache between updating firmwares.
- Repeat steps 4-7 with the nv64k.bin firmware file.
- Login and start tweaking
- Should you want to use DHCP ip assignment, remember to un-set the static ip that you set in the above instructions.
 Build Information
As noted in bold in the flashing instructions, only use builds with nv64k in the name for upgrading, anything else will brick your router!
To find the latest builds, you can access the DD-WRT file server. You will need to look for the broadcom_K26 folder, in there should be the appropriate nv64k build in which you can use for upgrading.
Good build for this device is 21676:
Forum thread for reference: