I have had a rd300 gx since 2009...love it. I recently played a friend's fp4. The fp4 is like new. Beyond the obvious difference due to years of use, the action of the fp4 seemed a little better to me, more consistent than the 300. The fp4 has the superior grand, but I couldn't find the Grand RD, which is one of the RD pianos that I like the most. In general, the RD's sound palette is much more complete and the options to regulate the sensitivity curve are also (the FP4 only has 3 options: soft, medium and hard compared to the RD, which has a linear curve and an exponential one, and each one has 90 variations. Excuse my English, I write in Spanish and translate with Google.