Debian – cron: can’t lock /var/run/crond.pid, otherpid may be 3759

It would appear that you are trying to run a cron job as your user, and that you have some form of encrypted home directory set up.

Your home directory (or some subset thereof) is unlocked and mounted when you log in, but when you are logged out, your home directory is unavailable, thus the error in your logs.

You should remove the encryption, run the cron job as another user, or you could try to run the job as your user but specifying a different environment (HOME, most notably).

Your syntax should be

0 */4 * * * <command>

A * in the minute field causes it to run every minute, as you're experiencing. A zero will cause it to run at 0 minutes past the hour, every 4 hours.

Not sure why you can't restart cron; perhaps your cron job is DoS'ing your machine.