Because you're using an $identifier where mIRC expects a /command, so mIRC tries to execute the return value of the identifier as a command.

So if $tip() returns 1, then mIRC will try to execute /1

To fix this, use the /noop command before $tip():

Code:
on *:TEXT:*:*: {
  if ($me isin $1-) { 
    noop $tip(Name,Highlight, $+ $me Je bent genoemd in $chan om4 $timestamp ,7200)
  }
}


/noop simply does nothing. So it lets you evaluate the contents of whatever you pass to it without doing anything with the results.