Have you trully put the thought into what 4 adult(if they get to that point) nile monitors will cost and the space and time they will take up plus if you don't even have your first two set up right yet why would you be trying to get two more to not set up right? Slow down get your current ones set up right and get them to adult size once you are dealing with two adults going full speed then deciede if you want two more. Trust me on this your trying to get too much to fast ...i've done this myself before it will end badly for you and your animals.
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"We don't inherate the earth from our parents, we borrow it from our children"