Re: Music theory nerd needed!
Well, assuming you want to stay in C Major/ A minor, you could add an A or a B to the first chord, a C, A or B to the second chord, a C, D or E to the third chord, and a D, F, or E to the fourth chord. Honestly, it's best to dick around a bit and find what you like, these are just examples of what would fit well with what you've already got.
If adding a third note doesn't quite do it for you, try adding a fourth. Making the first 3 chords, 7th chords, would probably sound nice.