Lily, one of the things you look at is "polyphony" (in the specifications).
This is the number of sounds you can make at any one time.
This might sound a lot, but when the keyboard is playing drums (each drum sound is a "note"), bass and backing, the notes get used up quite quickly.
The CTK-4000 has up to 48 note, the Yamaha PSRE423 has 32.
Now this might be offset by the fact that the Yamaha could have better sounding lead tones, so you don't have to rely on layering and auto-harmony as much....
The Y-man