This first post on my guide to hacking the TP-Link WDR-4300 shows you how to open the case.
Start by flipping the unit over so the top is laying face down. In the four corners your will see Philips head screws, start by removing these.
After you've done this flip the unit back over again and have the ports facing you.
Unscrew each antenna from it's lowest part in a clockwise motion.
Each antenna connector is held in by an outer nut, I used a pair of long nose pliers to loosen them half a turn then the rest was easily done by hand.
NOTE: Behind the nut is also a washer which so be careful you don't lose it.
After you have removed all the nuts and washers you simply push each connector in towards the centre of the case and they pop out.
Removing the lid of the router is hard to photograph so i'll try my best to explain. Flipping it back upside down again and looking at it from the bottom you will see a darker black outline that runs all the way round, this bit is actually part of the top of the case which clicks in the the bottom half. The idea is to get something thin enough between the dark black part and the lighter part, a thin flat screwdriver head/finger nails whatever you have handy. Then it's a matter of prying the top section away, I found I had to be quote forceful to get them apart.
After doing that you should hopefully be left with something like this.