I agree with you, that waiting until they are 4 weeks old is the optimum. I also uderstand that sometimes this just isn't possible if you need to move young puppies now in order for them to escape being euthanized.
One correction, most small aircraft do not have pressurized cabins. Some do, but the majority do not. However, with pups aboard, pilots may be able to maintain an altitude that will be condusive to good oxygen flow for the pups. Keeping them warm is definitely a priority and following your own vets advice is the safest way to go. Flying young pups is certainly less stressful then a long car transport, we just want to make certain everyone checks with their vet and gets the ok.