Workio DP-2500

E4-01 is the fuser not heating up in time. It can happen after paperjams, and usually it is no big deal. A reset of the code and you might be good to go.
Go in to service mode:
Function + Original Size + 3
Hit 9 for F9 mode
Scroll 'til you see Ram Initialization and enter there
Look for an "LBP Fuser Reset" option. Choose that and it will then ask you to turn the power off and back on. If the code comes back up then you have a blown thermistat or thermal fuse. You'll have to pull the fuser out to take care of it, but you'll want a tech to come out and do that.

There is a chance that you might have pulled out a sensor when taking out the jammed paper.
E codes are normally not an end user fixable problem. I am sure you have tried switching the unit off and on. Panasonic units sometimes have two switches for turning the unit off - one on the left side and another at the back of the unit.

I will try to see if I can find out the specific E04-01 code

the output of photocopy appears to be very dark, what shall we do, we don't know where the problem is, which part has trouble