Air units moving in the Combat move phase must have a valid landing spot available for each plane. This means with 1 carrier, you can move 2 planes (not 4) and count that carrier as there landing space providing the carrier and planes will have enough movement to meet. It is possible to send 4 air units into a battle because of 1 carrier if the ones that start on the carrier can land elsewhere to make room for up to 2 more planes.
The way I understand it, you can count the carrier being able to meet the planes even if enemy ships have to be attacked first to clear a path. I understand that the attack to clear must occur and regardless of the odds, you can then allocate planes to attack somewhere and use the carrier’s “future” position as a landing spot. If the naval battles to clear a path fail, then any planes that need to land on a carrier that can’t meet them will crash.
With regard to carrier designated planes, if they are all destroyed in combat, the carrier that was going to move to give them a landing spot in Non combat no longer has to move to provide a landing spot since the planes are gone. This is helpful when UK wants to destroy the Italian fleet at sz95 and then moves the carrier elsewhere if they lose the planes in combat.