Bad Day of the Month error in simple crontab
Please use crontab guru for creating and testing cron tab times.
Every 4 hours means using the HOURS column and dividing by 4 with the MINUTES set to a fixed amount. So that would be 0 */4 * * *.
*/240 * * * * would mean every 240th minute of an hour. That seems invalid to me as minutes end at 60 so 0-59 are valid numbers.