1998 Subaru Outback Engine missing badly

My outback started running badly in the last couple weeks - few hundred miles. Ran fine up until then. Now it won't idle, missing so badly at lower engine spees that it quits. Seems to run fine at higher engine speeds, kicks into passing gear ok and accelerates fine with pedal to the metal. The check engine light has been on and the OBD2 code reader indicates P0302, 3, 4 indicating misfire. I changed spark plugs which it needed badly but it didn't help. Seems unlikely all the plug wires would go bad at the same time, or coil etc. For that matter. Any suggestions? Could it be a fuel injection problem? What to do?

Thanks.