Can someone explain the difference between PTA and REX, and if they are both "needed" in P3Dv4?

Thank you

PTA allows you to alter the shaders in P3D. REX is a set of textures (for ground and weather). They are both quite different and can be used together. Neither are needed as such. I use PTA but not REX.

Have a look at this PTA vid.

And there's a big thread on weather here.

In addition to what Matt said: REX as such isn't a product but a company, fully named REX Simulations. And they mainly (but not only) create texture addons like Texture Direct, Soft Clouds, Sky Force. None of them though, as Matt said, are anything like PTA.