Nord sounds better but is dynamically idiosyncratic. For instance, the Kawai MP7SE plays dynamically a lot more like a new acoustic piano compared to the Nord, even though they both have the same action. The White Grand patch on the Nord sounds gorgeous... If only they fixed the dynamic control issues.
RD2000 has a comparable action. The modelled sounds are tweekable but the internal amp is not the best and I didn't enjoy the sounds in my test. Prior to testing, best to read posts about the issue and try to also test headphones that work well with the RD2000 or test a headphone amp in conjunction with the piano.
The RD2000 is more of a workstation style stage piano, with more functionality, whereas, the NP4 is more like a synth with regards to its user interface. On the RD2000, you would want to create registrations and switch between them eg, using a foot switch, as the sound selection method on the RD2000 is not user friendly. However, it has a very effecient effects matrix. Also, you can control organs with slider controls. The RD2000 has tones of sounds, so many that it's overwhelming. You can layer 8 internal sounds and you can control also 8 external sounds. Layering on the Nord is limited in comparison.
The NP4 gives you access to the Nord sound libraries. This means that you can upgrade your piano sounds when the company upgrades the NP4. However, there is limited memory for those sounds, so if you load tones of high quality piano patches, that might take up a lot of your available memory.
The Nord controls everything by switch or knob, so on the fly tweeking is easier than the RD. The Nord allows less layers.
The Roland is a lot less expensive and used deals are numerous. Not many sell their Nords, as they upgrade so well via the library.
I would definitely try the other available stage pianos, including the MP7SE, the Korg Grandstage, and SV2, the Dexibell VIVO S9 Pro, the Kurzweil Forte, and the Kawai MP11SE.
If action authenticity is less impactful for you, and if you like the NP4, then you might like to try the Nord Stage 3, which is more interesting apart from the action.