I still have my EMU E5000 here, and have to agree with Paul that the Emu and 4k engine are almost equally powerful, except for the fanatic Emu modulation matrix that mimics their good old modular period with patch cords. Fun!
The Emu has great (digital) filters, very warm with tons of character. Not really suitable for modern filter sweeps with lots of resonance, but perfect to color a sample. I tried the 4k filters (only got it a couple of weeks ago), and I was pleasently surprised by them, also very powerful in their own way.
Soundwise I think the Emu is more creamy and mellow sounding, while the 4k is more direct. The filters play a big role in that, but even without filters used in a sound, you still get that warm sound in an Emu. There's some coloring going on there in the Emu.
The Emu has a very high quality sample interpolation technology, meaning that you can pitch down a sample many octaves and still get a good sound without the tons of digital glitchy artifacts that you get from a Yamaha sampler when pitching down 1 octave. I use the Emu for super-slomo effects. Take any sample, pitch it down a lot, and enjoy a new world!

The 4k did a good job when I pitched down a sample by 2 octaves, but haven't gone to any extremes yet.
Finally, my Emu has the RFX effects card installed, and it's very nice, producing many different good quality effects. Only the reverbs are so so, the rest is great, imo. But is a bit complex to handle. The 4k effects aren't bad at all, I just went through them yesterday. I was afraid that the reverbs would suck, but they actually sound good when used with values near their presets: short to medium length reverb times. For reverbs with tails > 6 seconds you need something else. The other 4k effects sounded fine as well. Not very dramatic, but very useful.