Lightroom is the light-middleweight of Adobe products in terms of system requirements for a productive user experience . The CPU and Mobo are far more than adequate for handling the processing tax that Lightroom imposes . If you were routinely processing hundreds of raw files under a production deadline, I might suggest a 6 core processor , but ..
You should follow the recommendations and go to 16 meg of memory . I'd also reevaluate your disk configuration . One more SSD , and another faster platter ( 7200 or 10000) would help .
A discrete graphics card helps in two ways . The biggest way is it will not require the 500-750 M of dedicated memory ( HD 4600 takes about 700 M) , which frees this up for your application. The second is that, while Lightroom for the most part will not require nor offload processing to the graphics card as Photoshop or Premiere will for some filter and effects processes , LR 5.5 does leverage this if one is creating video slideshows, and a few other tasks . I haven't found all the doc on it , but it's leveraging parts of the Adobe Media Decoder, and maybe fragments of the Mercury Playback/processing in use in their other products . Discovered this accidentally while running process explorer and GPU-Z on a laptop with Optimus ..
Thanks for the suggestions!
I do routinely process hundreds of 22 megapixel RAW files under production deadlines

At least once or twice a week. And I do local adjustments too, not just RAW development. In fact I rarely use Photoshop anymore.
But I don't think that a six core processor would result in much of an increase of performance and it definitely would not justify the cost. The truth of the matter is that at this point of the game it doesn't make any sense to try to solve a software problem with hardware tools. Adobe just need to get their act together and make the damn thing utilize hardware resources better.
I did upgrade to 16gb of RAM, although I've done some tests over the past few days and it barely makes any difference - maybe 1-2% increase in performance, but that's within the bounds of statistical error. I don't think I've ever managed to fill-up the memory and start offloading to virtual memory, even after hours of work and with Lightroom's notorious memory leaks. So I also really doubt the 500-700mb of memory freed up by an external graphics card would make any difference whatsoever.
Regarding slideshows: I rarely make any in Lightroom, but even when I do, that's a batch job that I can quickly set-up, let rip and go to the gym or have a coffee. Plus, considering the trivial complexity of the effects available in Lightroom, I seriously doubt a discreet card would make much of a difference. Maybe it will speed things up 10-15%, but it would cost a good chunk of money + additional heat + another fan + additional power consumption. I'll get one when Adobe offload preview creation and adjustments in the Develop module to the GPU. Then it will be worth it, of course.
More and/or faster disks, even SSDs, beyond one fast SSD containing the catalog, cache and preview data don't make any difference. Take a look at the articles I linked above. Adobe don't believe it does themselves and I've tested it myself too. It simply does not improve anything. As long as the catalog, cache, previews and the files you are working on are on an SSD, you are good to go.