Control unit defective!?

Hello together, my Ibiza 1.4MPI (1998) runs only on three cylinders. After a long search I found out that the injector no longer closes (the thing that injects the gasoline hot nevertheless injector? ) and thus all the time got petrol. I had the connection / plug measured times and it clocked first, then again plugged up nevertheless no change. Again from and measured, now was only a continuous signal there. Does that mean that the control unit is defective or can still i anything between the plug and the control unit cause this error? If it is the control unit, are there still economically sensible ways to fix anything? Just put the used one on it probably doesn’t work, doesn’t it? Finally another question, if my darling is no longer to be helped, can I just drive on three cylinders (tank is still full)? I pulled off the defect plug, so he no longer injects gasoline there, so can The cat doesn’t fly around my ears, right? But I already drove about 50-80km with it until I have noticed what thing is. Thank you very much! Greetings Ludi