We have faced error E4 on Canon printer MP145 by yesterday. The error occured after refilling the cartridges (black and color). After re-install the cartridges into the printer, we turn on the printer and the the error code E4 displayed on 7 segment LED indicator.
There are three buttons on the printer, that are black, color and stop/reset (look at the picture). Simply try to press one of them for about 5-10 seconds. Your printer should be work correctly now… It may be work for other PCS (Print Copy Scan) printer types of Canon.