Hi!
Nope, you can't "MIDI dump" sounds from a Motif, Triton or whatever to the 4K. If you want the sound of one of these keyboards in the 4K you will have to sample them. The 4K is, after all, a sampler.
The reason is quite simple, MIDI doesn't carry any sound information AT ALL. It's just notes, and descriptions on how these notes will be played. Nothing more. The Motifs and Tritons of this world have large amounts of resident sampled sounds that make up their programs/patches. You can't just "dump" this information over to the 4K, 'coz it doesn't have these resident samples, it has another hardware architecture all together. Sure, you could sample the raw waveforms that make up a Motif or Triton meticously to reconstruct the sounds, but even then it wouldn't be quite the same thing...
Anyway, if you're in need of some nice instrumental samples for your 4K, head over to
http://www.hollowsun.com, they have a large collection of nice, free soundbanks for the S5/6000, Z4/8 and 4K. Mostly vintage gear, but some usable electric pianos and "updated" sounds too. Check it out.
Hope this helps,
Arne S.