All answers seem to focus on the nature of random while the question concerns truly the nature of knowledge. What is it to know something?
You seem to implicitly allude to an unattainable and impractical notion of knowledge, some God-like insight into matters. We're human, for us knowledge is not an absolute state of clarity and vision. We know that it's Friday today, that milk is white and the Winter is coming etc.
Unfortunately the subject is outside the field of statistics. Hence, my terse answer: if your RNG keeps returning 4, then you will know that it's not random after a few trials.
You and I know that the Sun will rise tomorrow. If someone doesn't then they should see a therapist to deal with anxiety, maybe take some pills etc. The point is that it's not the subject of statistics in this case.