http://www.emulatorarchive.com/Samplers/E4Overview/E4Ultracize/e4ultracize.html
In January 1999 E-mu Systems announced that some later models of the Emulator 4 range (E4XT, E4X, E-Synth Rack, or e-6400) can be updated to become a full function Ultra. This entails a motherboard swap at a service centre or E-mu Systems, and it costs from around $995 - $1395.
The upgrade is $1395, but you get a rebate of up to $400 if you have a high specification model already.
Who is Powered Off ? If you own an E4, an e-64 or any of the E4 keyboard versions - you can not upgrade. The necessary hardware changes are so expensive, that it is cheaper to buy a new Ultra.
Should I do it ? If you really want the new Ultra power and if you haven't spent lots of money upgrading your current Emulator 4, then go do it. However you might want to consider getting an E5000 as it is not much more than the upgrade. At least the sample RAM SIMM's can be re-used on the new board.
The full set of Ultra features is as follows:
The Motorola 5206e ColdFire RISC 32-bit processor
128-voice polyphony
The existing GO5 24-bit FX engine
Dual MIDI In, Out and Thru
Word Clock In and Out
Digital AES/EBU ASCII interface
4 MB Flash OS, upgradable to 16 MB
20-bit DAC's with 128x over-sampling
8 balanced outputs, upgradable to 16 balanced outputs
EOS 4.0 (Ultra version including Beat Munging)
E4 Ultra Production Sound set CD-ROM
2 RAM, 4 ROM slots
Internal upgrade slot location for the upcoming RFX-32, 32-bit FX processor
MAC/PC version of EOS Link, remote control software