  We discuss the spectrum and the internal composition of ground and excited
four-quark states in the charm and bottom energy region. To this end we extend
previous calculations within the framework of the relativistic four-body
Faddeev-Yakubovsky equation to include quantum numbers with $J^{P C} = 0^{++} ,
0^{-+} , 1^{--} , 1^{+-}$ and $1^{+ +}$ and study their internal composition in
terms of heavy-light meson pairs, hadroquarkonia and diquark-antidiquark
clusters. We observe similar patterns in the charm and bottom energy region
with different compositions of the four-quark states depending on $J^{P C}$
quantum numbers. Most notably, we find that all states with $C \cdot P = +1$
are dominated by heavy-light meson contributions, whereas for axialvector
states with $J^{P C} = 1^{+-}$ including the $Z_c (3900)$ we find a much more
complicated picture depending on the flavour content. We systematically compare
our results for the spectrum with existing experimental results and provide
predictions for future analyses.
