I use Lightroom 4.3 on my Surface Pro (and also on my 17" MacBook Pro with 16GB of RAM). It's surprisingly fast and crunches through my Lumix GH2 RAW files quickly. Faster, in fact, than I remember Lightroom 3.x running on my Mac. It's a perfectly serviceable companion computer for Lr to keep in your bag. I was very hesitant about the 4GB going into it, but Win 8 seems very efficient compared to 7, so it doesn't seem like it's a problem yet. I'm still going to continue using the MBP for my main 12,000+ photo catalog, so I can't speak to how the Pro runs with a dumpload of images, but it's totally fine when I offload 40 or 50 at a time. It actually even runs Photoshop CS6 faster than I would have thought. I was able to quickly open a 5000x3000 25-layer 300MB file and very quickly and smoothly pan around on it, and draw over the top (no pressure sensitivity, though, but that's a driver issue that I think Adobe is working with MS on). Rather than RAM, the main concern would be battery life if you're going to use it in the field a lot.
*edit* I just saw that you intend to use it as a desktop replacement. I'd suggest a USB3.0 RAID or external SSD to keep your main catalog on. 128 goes fast when you're dealing with RAW files.