I am glad the WNDR4000 works so well for you Charlie.
I suspect for me, since my E4200 has been performed flawlessly (since turning off my optional Rosewill wireless utility software which caused the symptoms you had with your second E4200), that I won't see any benefit.
I may order the WNDR4000 just for kicks to compare it to my E4200, but I doubt the performance will be any better.
I would not be surprised if the performance of the two, for me, is virtually identical given the Broadcom chips are the same. If not, I guess the firmware of one has tweaks the other doesn't...
I'll post again with comparisons if I do get a WNDR4000.
Still deciding if I want to bother getting the WNDR4000 given the excellent performance (getting a great wireless signal on 2.4 GHz), reliability (in terms of maintaining wireless connections without a single dropout) and signal quality (pingtest.net results are excellent, even when using my wireless connection) of my E4200 so far.
If they sold the WNDR4000 in retail stores, I would definitely give it a shot, but having to return it to newegg may not be something I want to bother with (i.e. if it performs no better than my E4200 then I would rather keep the E4200 given the unique B&O-like looks Tim cited).