Hi, I am using 40mm on some tanks and no issue. You can push a lot of water through 40mm!! Bridging is used on tanks on same level, if you want to use this for 2 tiers, then tier 1 will be bridged as a unit and tier 2 as a unit, if you wanted to share water between the 2 tiers you would have to make some modifications by having a "overflow system" to feed the bottom tier and then use canister to feed up to tier one. 4ft tanks are fine to use, it is really about the tanks being the same height that are being bridged, the size of the tank does not really matter.
Yes the bridges create self regulating / leveling.
There will be a point where the bridge diameter will be to big and the water will fall out of the bridge, but you should never get to that point.
If you have concerns get 2 cheap plastic containers (same height), build one bridge between the 2 and if you have a spare power head, get it to pump water from one container to the other. Then you can see if the bridge is doing what you want without spending lots of cash.
@aquaholic99, has used this system on more than one tier and can provide some pics if you ask him nicely. :-)