  We forecast constraints on minimal model-independent parametrisations of
several Modified Gravity theories using mock Stage-IV cosmic shear data. We
include nonlinear effects and screening, which ensures recovery of General
Relativity on small scales. We introduce a power spectrum emulator to
accelerate our analysis and evaluate the robustness of the growth index
parametrisation with respect to two cosmologies: $\Lambda$CDM and the normal
branch of the DGP model. We forecast the uncertainties on the growth index
$\gamma$ to be of the order $\sim 10\%$. We find that our halo-model based
screening approach demonstrates excellent performance, meeting the precision
requirements of Stage-IV surveys. However, neglecting the screening transition
results in biased predictions for cosmological parameters. We find that the
screening transition shows significant degeneracy with baryonic feedback,
requiring a much better understanding of baryonic physics for its detection.
Massive neutrinos effects are less prominent and challenging to detect solely
with cosmic shear data.
