The precipitation state of an Al-4 wt% Cu-1 wt% Mg-Ag alloy reinforced with SiC particles fabricated by squeeze casting is studied by transmission electron microscopy. As a consequence of an interfacial reaction the precipitation of the characteristic phases of the unreinforced alloy, Ω and S′, is suppressed and substituted by a new precipitation consisting of isolated nanoscaled rod-shaped precipitates and θ′ plates decorated with a further type of rod-shaped precipitates. These precipitates are the Q-Al5Cu2Mg8Si6 phase (hexagonal with a = 1.035 nm and c = 0.405 nm), and two other new phases also containing Al, Cu, Mg and Si, called QC (hexagonal with a = 0.670 nm) and QP (hexagonal with a = 0.393 nm). These three phases are based on the same latent lattice, the QP lattice. A phase transition model is proposed for these phases. It implies an ordering of the structures given by displacements of atoms from a lattice based on the common QP lattice. This approach is very promising for a better understanding of the precipitation in Al-Mg-Si-Cu alloys.