What should I be learning to be able to write decent melodic songs

[Robguitaruk]

I know this is a broad question but I struggle to make anything but a 4 chord sequence with a simple melody. I will say I am quite new to composing and I am mainly a self taught guitarist (though can play a little piano). When I listen to other music the melody never seems to be this contrained and there is almost always movement away from a rigid structure.

How can I go from 4 chords to something that is more alive. Like any Disney song, or anything by John Williams or many many others. The music and melodies feel so alive within these types of pieces and I would love to be able to compose like this but I have no idea how to go from 4 chords to something more alive.