If you're using this PSU (
http://www.mini-box.com/picoPSU-160-XT) then all you need to do is split all 4 wires into 8 (4 yellow [12V] and 4 black [GND] and insert them into the new 8pin EPS connector the same as the 4pin was (yellow [12V] on the top row [near the clip] and black [GND] on the bottom row)
as for splitting the wires, the "pigtail" method would be sufficient (since it's a low wattage application anyway) so you don't have to try and solder on 4 more wires. I'd just cut the old 4pin connector off and crimp the new terminal/pin to the original wire and an additional 'jumper' wire (recommend 18guage or larger), crimp the other end of the 'jumper' wire, and insert into the new connector.
Should be pretty straightforward
- let me know if that didn't make sense.
Good luck!