Hello,
I'm looking for a stage piano to continue my studying of classical piano while I'm not at home. I'm not sure which one to pick: possibilities are yamaha P140, Roland RD 300SX and Roland FP4
I haven't tried de FP4 yet and only found it on the internet with a description matching perfectly what I was looking for.

What can you say about the action? My objective is to buy a piano which is as similar as a real piano as possible. I.e. not to light action, good piano sound, no extras needed. I would be playing with headphones mostly.

I felt the roland 300 could be a little light, not having optimal effect when studying for technique maybe?

thanks already!

Do you consider having the piano at the right height and stable enough important - if so, consider a serious stand designed for the DP you selected. Get a bench too.
Also, the piano you listed come with 1 piano style pedal (at least I am sure the p140 comes with one). Other DPs might not include anything but a cheap plastic pedal.
Also you said that you want good piano sound, and based on the experience I have with the P-120 (that I love), the best sound is experienced with quality head phones (I mean something that does not amplify the bass and has a pretty flat response on every range) - the internal speakers are doing their job but I consider them weak in the low range - I have not been more impressed by the FP-4 in this area, I don't recall anything spectacular from teh Roland either.
It adds up at the end. Consider it.
The pianos you listed are all excellent choices, and you should spend time to try them as it will be at the end a matter of your personal taste.

I would avoid the Roland rd-300sx. The action is feather light. Too light in my opinion for classical piano work.

If you're near a big city go to a large music store and try every brand they carry. I'd suggest also trying the Kawai and Casio. Touch is personal, buy what feels good.

Do you have a price range?

I would add the Kawai EP2 to your list.

Video Demo: http://www.kawaius.com/video/EMI/ep2_winvid.html

I'd say consider the Roland FP-7 as well. I thought the action was the best of all the actions I tried, and is very close to a quality acoustic. And keep an eye out for a stable stand (some Quiklok stands come to mind) and either an adjustable bench or a stand that is adjustable, height-wise.
Good earphones are great, unless your budget allows a separate amp and studio monitors.

But really, try every keyboard you can, consider expanding pricerange (almost always can get more for more money) and keep in mind that you can often get good instruments either used, on sale, or just for much cheaper than advertised.

Personally I prefer the action of the P140 or FP7 to the FP4, which feels too light and not as sturdy as I like. Between the P140 and FP7, my subjective impression is that the P140 is firmer and a bit more like an acoustic. As others have said, the more keyboards you try, the more you'll know what you yourself like.

Don't be misled by the argument, when it comes to actions, that heavier is necessarily better because it's "more like a real piano". The more grands you play, the more you realise that very few of their actions are set as heavy as a typical Yamaha digital stage piano, for example, that of the CP33.

This is why it's very important to play candidates yourself. In my experience, how satisfying a DP feels to play is more closely related to key "balance" under your fingers rather than the weight itself.

I recently did a gig where I played three pianos in one engagement - a Broadwood grand, a Steinway grand and my own Roland FP-4 digital. There was no substantial difference in the overall weight of the actions - the Steinway was a bit lighter than my Roland, if anything. But the two acoustics felt different because of their mechanisms and sonic connection. You can't realistically hope to find that in a digital.