I did use 4 LEDs. I had to trim away part of the cup holder where the bulbs sit to get them in there. I used 1/4" tubing for the diffuser just like Clean13' did. I used the same LEDs as him, too.
Here's some bad cell phone pics of a test fitment. I was able to get all 4 in there.
As you see, I tested using a 9-volt battery to test, so they will be brighter after permanent installation.