You can’t make it “generate” only higher severity messages, but you can reduce the volume of messages that get placed in the ‘joblog’.
First, instead of LOGLVL(4 00 *NOLIST), try LOGLVL(3 00 *NOLIST) or even (2) or (1). Level (4) says to log every kind of message possible. Set a logging level that you want.
And instead of LOGLVL(4 00 *NOLIST), try LOGLVL(4 30 *NOLIST) or a higher number. Severity (00) says to log every severity from 00 up to 99. A higher number says to log only messages of that severity or higher.
However, the best choice is not to have the messages at all unless they are <i>intended</i> to be logged. The messages should log information that needs to be known after the job completes. If it isn’t needed in a joblog, it shouldn’t even exist. It probably shouldn’t have been sent.
If these are coming out of API calls, then the programming should be changed to avoid having them from being sent. API error code parameters generally should be used instead of using messages.