The core hypothesis of 4D Theory (4DT) is that our universe exists on the surface of a four dimensional object that exists in four dimensional space. The implications of this hypothesis are varied and profound, but the most interesting and complex implication is the emergence of a new Aether in the form of the surface of the four dimensional object.
The original Aether theories were all designed to explain how light existed since light was a massless wave.
A wave is defined as energy passing through a medium. A wave has no mass because it does not exist as a thing in itself, but rather as a momentary characteristic of another thing. For example, a sound wave is a compression of particles. The particles exist as themselves. A sound wave is the momentary compression of those particles.
The problem with light, though, is that it passes through a vacuum. A vacuum, by definition, has no particles and no substance beaning that there is no known medium in a vacuum for light to travel through.
The Aether theories of the past were designed to explain the propagation of light using the concept of "aether" to mean some exotic substance or field with could exist within a vacuum. The famous Michaelson
Einstein famously rendered the concept of Aether obsolete through his Theory of Relativity.
Less famously, Einstein actually believed that the Aether existed but that his space-time was the "new Aether."
Redshift due to time dilation is called the Relativistic Doppler Effect.