Welcome to the forum
I found another forum where this answer was presented:
E4 is error on optical pickup. One or two of the problems might be. You disc lens is out of alignment, or the discs your using are the wrong type. You need to use a SOLID SPEED DISC. Yes:Dvd-r x4, x8. Dvd-rw x4, 8x. DONT use up to 16, 1-16x or antyhing with UP TO or 1-8x, 1-16x.
From what you have said, it seems the drive may be going bad. IF you have been trying only 4x/8x discs? There are many DW-1650 clones out there, and you should be able to get one reasonably cheap off E-bay. Help with flashing a Philips or other DW-1650 equivalent can be found in the BenQ sub-forum. As well as a list of all the clones out there you could use as a replacement.