The baby looks fine, he's just not as grown up as the others yet. The "different" baby is pearl pied and the other three are just pied. The pearl baby is definitely female - you need a pearl mother to get a pearl son, and mom isn't pearl. The non-pearl babies could be either sex.
You can keep the nest bedding at a level where the babies can look out the door but have to climb up to actually pass through the doorway. They'll be pretty well feathered by the time they're able to do that and no harm will be done if they tumble out a little earlier than they planned. You can just put them back in the nest from time to time and let them decide if they want to come back out or not.
Changing the cage around probably won't make the parents start plucking the chicks. If they thought the area around the nest had become dangerous they might start plucking to try and get the chicks to leave the nest, but cockatiels are fairly adaptable and they probably wouldn't respond this way. They're more likely to start plucking if they want the babies to leave so they can start a new clutch, and rearranging the cage may throw them off enough to not want another clutch right away.
Some parent birds will start laying eggs while the previous babies are in the nest and don't mind the older babies being there, but most of them prefer for the older babies to leave. It will depend on the personality of your parent birds. The mother is usually the parent who's most likely to pluck and your hen is acting like she wants to double clutch, so she's the one you have to watch.
If you have the money and space for it, getting a bigger cage and moving the nest to that would be better than rearranging the current cage. This is a great cage for the price:
http://www.amazon.com/Prevue-Products-W ... B00176F5L0 and there are three small doors on each side plus two big doors on the front so there are plenty of options for hanging a nestbox on the outside. There's a bigger version of this cage too:
www.amazon.com/Prevue-Hendryx-F050-Prod ... 002AQ228/r The food cups that come with it are sucky so you'll probably want to replace them with 'coop cups'. If you decide to get a bigger cage, it would be less disruptive if you let the parents spend some time in it for a day or two to get used to it before you move the nest.