Apex cron job command to schedule an apex job to run every 4 hours daily

If you use the following chron string for your scheduled job, it will run every 4 hours (at set times) Monday through Friday.

apexScheduledJob j = new apexScheduledJob ();
String sch = '0 0 0,4,8,12,16,20 ? * MON-FRI';
System.schedule('My Job', sch, j);

If you wanted it to run every day, you would just change it to

String sch = '0 0 0,4,8,12,16,20 ? * *';

use this site to make cron for every scenario. cronmaker