In a 4 person game where B,C, and D control Notion Thief and A draws a card what will happen is A will choose which replacement effect to apply (player B let's say). Then player B will go to draw, but there are still two more notion thiefs so Player B will choose which one to apply next (Player C) and then finally player C will have their draw replaced by Player D. Because this is all happening from the same original event each replacement effect can only apply once so each player will essentially be choosing someone who will not get to draw.
For each card drawn by the person without a Notion Thief (outside of the first they draw during their draw step) they choose which Notion Thief to apply first. So, if Player A is drawing a card and Players B and C have a Notion Thief, they can choose which one applies first. Let's say they choose Player B's replacement effect to apply. Then, when it applies, the other Notion Thief still applies since that player's opponent is still drawing a card. In that case, Player C draws the card.
If player A chooses to apply player C's effect first, Player B will ultimately end up drawing the card.
Note that If player B or C starts the process by drawing a card, the two Notion Thieves effectively cancel each other out.
What happens if multiple people have Notion thiefs and then one players who doesn't have a notion thief draws one too many cards?
If player A chooses to apply player C's effect first, Player B will ultimately end up drawing the card.
Note that If player B or C starts the process by drawing a card, the two Notion Thieves effectively cancel each other out.