  We investigate the median flattening of galaxies at $0.2<z<4.0$ in all five
CANDELS/3D-HST fields via the apparent axis ratio $q$. We separate the sample
into bins of redshift, stellar-mass, s\'ersic index, size, and UVJ determined
star-forming state to discover the most important drivers of the median $q$
($q_{med}$). Quiescent galaxies at $z<1$ and $M_{*}>10^{11}M_{\odot}$ are
rounder than those at lower masses, consistent with the hypothesis that they
have grown significantly through dry merging. The massive quiescent galaxies at
higher redshift become flatter, and are as flat as star forming massive
galaxies at $2.5<z<3.5$, consistent with formation through direct
transformations or wet mergers. We find that in quiescent galaxies,
correlations with $q_{med}$ and $M_{*}$, $z$ and $r_{e}$ are driven by the
evolution in the s\'ersic index ($n$), consistent with the growing accumulation
of minor mergers at lower redshift. Interestingly, $n$ does not drive these
trends fully in star-forming galaxies. Instead, the strongest predictor of $q$
in star-forming galaxies is the effective radius, where larger galaxies are
flatter. Our findings suggest that $q_{med}$ is tracing bulge-to-total ($B/T$)
galaxy ratio which would explain why smaller/more massive star-forming galaxies
are rounder than their extended/less massive analogues, although it is unclear
why s\'ersic index correlates more weakly with flattening for star forming
galaxies than for quiescent galaxies.
