Connect an external keyboard and see if the issue continues. If not the keyboard is at fault. if the issue remains then boot into safe mode and see if the issue persists in safe mode. If not then it is a software issue with a program in Windows 10.
Hi, Sorry for the delay in replying - I was trying to get hold of a USB keyboard... When I (finally) got hold of one (and disabled the built-in keyboard) I still got the random "4"s and left arrow key inputs (actually, I still got them with no operative keyboard, USB keyboard unplugged and built-in keyboard disabled). So, it looks like this isn't a hardware problem. When I rebooted in safe mode there was no issue. So, if it's a software problem with a program in Windows 10, how do I narrow it down to the particular software? Thanks