I think E2 means open circuit in thermocouple in center of bottom. E4 means no contact on pressure sensing switch at edge of bottom. Mine had E4 error. Took out switch and used fine sandpaper to clean contact points. Now it works fine.
Remove 3 screws in bottom. Remove bottom cover by turning it ~1/4" to clear tabs. Remove switch at edge of bottom with two blue wires going to it. Clean contacts and replace.

E4 or E-4 error message in the Wolfgang Puck Pressure Cooker usually means that the pressure cooker is not building up pressure, meaning it is not sealing properly.

This is frequently caused by the pressure relief valve not properly seating. Usually you can get the relief valve to seat by putting a little pressure on the top of the valve and wiggling it around. This will rub off the layer of minerals that have built up on both the valve seat and the valve needle. If that doesn't work, pull the top of the valve out of the seat, and manually scrape off the buildup of the mineral buildup.

Not properly sealing can also be caused by a mis-alignment of the pressure cooker gasket seal. It sits in the lid part of the cooker. It can become a bit stretched. First note the location of the gaske, then take the gasket out and gently clean it. Put the gasket back in the lid and carefully put the lid back onto the pressure cooker. You can tell that it is properly seated if there is no gap between the lid and the pot. If there is a gap then the gasket is misaligned and so the lid is not seating properly.

Once you get the pressure release valve, and the gasket seal properly seated, unplug the pressure cooker to clear the E4 error message and try using the cooker again. Make sure there is liquid in the pot so that there will be steam to cause pressure.

Once the timer clock starts ticking down, there should be very little steam coming out of the cooker. Your pressure cooker should work fine now.

If not, you can try the methods described in other answers.

Omg..Eric..you were right on the money with your instructions...I had my pressure cooker for probably close to 3 yes or more and this was the first time the e4 code came out..I did other people recommended and nothing...I follow your instructions and bingo...I'm am cooking my chick peas right now...thanks a million...I was bummed out thought I had to go to buy another one and I surely am tight in money...thanks again!

I have been helped by the solutions suggested above, just got to contribute something.

I got this E4 code today, the pot is cool, there is no water inside, don't know what to do with it.

inspired by the solution above, I opened the bottom lid, make sure you unplug the power cord. Only three screws, very easy to open.

You will see one metal switch at the edge, and two blue wires connected with that metal switch. Just press the white button of that switch a few times you should hear little click sound and feel the switch been pressed down.

After that I just put the bottom lid back and plug in power cord. And start cook, it works fine.

I notice that there is some old cooking stuff already dried on that switch, that means sometime back I must have some spill over which got liquid entered into the bottom compartment. I cleaned that as well.

SB37438 has a good idea there.

I was able to bypass the E4 error by completely bypassing the electronics and wiring the heater coil to get power whenever the pressure cooker is plugged in. It was a very simple job, anyone with any electrical experience can do this.

The only problem is that it is a safety concern as there is no way for the heater to turn off even if pressure is getting to be way too high. There may be a pressure relief valve in the lid but the heater coil won't shut off, so don't leave it unattended! But at least this way to you can still pressure cook.
Pics of process:
http://s909.photobucket.com/albums/ac295/danwat1234/fixing%20the%20pressure%20cooker/

I need instruction on how to operate and set up