  We study the internal structure of a range of four-quark states with charm
quark contributions using a two-body Bethe-Salpeter equation. Thereby, we
examine charmonium-like states with hidden charm and quark content
$c\bar{c}q\bar{q}$, open-charm states with quark content $cc\bar{q}\bar{q}$ and
all-charm states with $cc\bar{c}\bar{c}$. In particular we study the internal
competition between meson-meson components and diquark-antidiquark components
in the wave functions of these states. Our results indicate that the
$\chi_{c1}(3872)$ and the $Z_c(3900)$ are predominantly $D\bar D^*$ states and
that the recently discovered open-charm state $T_{cc}^+$ is dominated by an
internal $DD^*$ component. In both cases the diquark components are negligible.
For the all-charm state $X(6900)$ with as yet unknown quantum numbers we
identify candidates in the excitation spectra of $0^+$ and $1^+$ states.
Furthermore, our framework serves to provide predictions for further, yet
undiscovered open and hidden charm four-quark states.
