The Electro 4D is an excellent option if you want to sound great and be extremely mobile. The Hammond emulation sounds extremely accurate to my ears, along with a very detailed and tweakable Leslie simulation. The Vox and Farfisa models are fun to play and they are a great option if you want to adjust the drawbars to create cool new organ tones. Splitting the keyboard is quick and easy to do on the fly and that makes this keyboard a lot more versatile.
The pianos are, in my opinion, the weak link on the 4D. I had to download a different piano tone from Nord's website to get one with which I was satisfied. After some EQing I was able to get a decent grand piano sound out of it, but I still like the pianos on my Yamaha M08 better. The waterfall keyboard action takes a bit of getting used to if you want to play highly dynamic piano music on the Electro 4D.
Some of the Rhodes electric pianos on the 4D also sounded less than satisfactory to me. They are all very different tonally from each other, so I suggest listening to them yourself, you may find one you like. My biggest issue with some of the Rhodes tones on the 4D stems from their individual "clicky-ness" and the tinny "bell-like" overtones present. While these may be faithful to an original Rhodes, I still prefer a warmer, rolled-off tone, which is easy to achieve by utilizing the 4D's fantastic built-in EQ, and Nord has some warmer sounding Rhodes samples available on their website. I guarantee you will find a tone you will like, however it may not already be loaded on the board, so I suggest going to Nord's website and having a listen.
The one Wurlitzer sample on the board sounds great to my ears, and I enjoy playing it. I especially like to run it through the onboard "Twin" speaker cabinet simulation and put some drive on it.
The clavinet samples are by far the best I've ever heard, and they are awesomely tweakable with the onboard effects. I particularly like using the wah effect which is controllable via an expression pedal (auto-wah is also available.)
While the orchestral and mellotron samples may be great-sounding and useful to another player, I deleted all of them to make way for synth samples downloaded from Nord's website. The synth samples sound great, but this is the one area in which I think the hardware of the Electro 4D fails; there's no pitch or mod wheels. Say goodbye to awesome synth-pad filter sweeps, or funky pitch bending solos. That said, the synth tones available from Nord's website are still top-notch and extremely varied.
The onboard effects on the 4D sound great. The tremolo, pan, phaser, flanger, and chorus all have three depths available, and can be adjusted via the continuously variable rate knobs. Some players may complain that two of their favourite effects are relegated to a single knob, and therefore can't be used together, but I think those same players are the type of who think having a bunch of effects on can mask their inability to actually play music. Not having phaser and chorus on at the same time doesn't bother me at all.
The onboard delay and reverb sound excellent. They are both highly adjustable, and the tap tempo button is a great feature for adding some last minute delay to a sound while on stage.
The speaker cabinet emulations and drive are very useful and sound great. The different speaker cabinets are all suited to different tastes, so while you may hate one, you may love another. The Leslie simulation is excellent, and I particularly enjoy using the "Twin" speaker cabinet with electric pianos, or if I want to get a Machine Head era Deep Purple Hammond sound (tip: If you're looking for Jon Lord's Space Truckin' solo sound from Made In Japan, the onboard ring modulator, combined with some drive and reverb, will take you Space Truckin' and then some.)
The build quality of the Electro 4D is astoundingly good. The buttons feel good, the knobs feel amazingly tight and sturdy, the drawbars have a satisfying stiffness and click to them. The waterfall keyboard feels awesome, although it may take a pianist a while to get used to the different feeling action.
I found Nord's sample manager software to be reliable and user-friendly, and I had very little difficulty in dumping sounds I didn't want and loading sounds I did want without reading any manuals or instructions.
All in all, the Nord Electro 4D does a fabulous job at recreating classic electromechanical keyboard instruments, and it is a gigging keyboardist's dream weighing in at around 15 pounds. If you want to sound great, and be able to make it to every jam session and gig without having your back give out, the Nord Electro 4D is for you.