We present the synthesis and structural characterization of new nanocomposites made of linear tetramers of spherical magnetic nanoparticles of maghemite (gamma-Fe2O3), grafted by tethered polystyrene (PS) chains with an intermediate grafting density (similar to 0.15 chains/nm(2)), dispersed in a PS matrix. First, we studied by combination of SAXS and TEM the dispersion state of the grafted objects within the matrix for various ratio R of the mass of the grafted chains N to the mass of free chains P ranging from R = N/P = 0.09 to R = 2.83. For R < 0.4, we obtained ramified aggregates of a few tens of grafted objects whose compactness slightly depends on R. For R = 1, the objects are well dispersed, a case that we have studied by recovering the free untethered residual chains resulting from the synthesis of the PS grafted nanoparticles. For R > 1, the objects are also well dispersed although there are some remaining aggregates, arising probably from the fact that we have used matrix chains below the entanglement mass to reach such a high R values. This enabled to determine the threshold of the "wet-to-dry" transition, between the wetting of the brush by the free chain and the expulsion of them from the brush due to entropic effects, between 0.4 and 1, a value higher than the one (0.24) obtained on a very similar system on silica spherical particles by Chevigny et al. [Macromolecules 2011, 44 (1), 1227133]. This shift highlights the influence of the curvature of the grafted objects on the threshold's transition. Second, we have made a direct measurement of the conformation of the grafted chains in the case of aggregates, benefiting from the fact that the scattering length density (SLD) of maghemite has almost the same value as the one of deuterated PS chains. They are strongly collapsed in comparison with their conformation in bulk, as shown by a significant reduction of their radius of gyration accompanied by the deviation from a Gaussian behavior. Finally, by applying a magnetic field during casting on samples for R < 0.4, we showed that the aggregates can be aligned in the direction of the magnetic field. We demonstrate thus the possibility of tuning the structure of the fillers by coupling a double control: chemical and magnetic.