I changed the alternator now I have a low idle

Hello, have you verified the battery is fully charged and able to handle the load of the vehicle? Low battery voltage can cause many different concerns, even if the charging system is working properly. And, have you verified alternator output - in voltage and amperage? Is it sufficient? What was the original cause of replacing the alternator? Diagnostic trouble codes would be present for a 4x4 light being on. They would be "C" related chassis codes, so the right diagnostic tool with full capabilities would have to be used.