A Chord is is recorded in one track, but it uses up to all the four voices, of course. As the keys has LEDs on each key, when you press the step the chord is recorded on you will see the notes the chord consists of. Very nice feature that I do not wanna miss!
The Arp takes the notes of the chord and plays them in an arpeggio, as expected. Additionaly there is a polymode on the arp that plays the whole chord instead of the arpeggio. I have yet to find a use for this feature. 
Yes, the keyboard is completely replaceable by a midikeyboard. But finding one with the features of the KEYS will be difficult.
Velocity
Aftertouch
LED per Key (I think this is not replacable)
Joystick
And when you buy the Analog four instead you don´t get the Multimap feature, I think, though I might be wrong.