Dynamics of the Energy-Critical Nonlinear Schr\"{o}dinger System in ${\mathbb R}^{4}$

In this paper, we investigate the dynamics of radial solutions at threshold energy for a 3-component Schr\"{o}dinger system with cubic nonlinearity in four dimensions. The main difference from the cases previously addressed in the literature is that, in our system, the kernel of the imaginary part $L_I$ of the linearized operator $-i{\mathcal L}=L_{R}+iL_{I}$ has dimension 2. To overcome this difficulty, we carry out a detailed study of the coercivity properties of these operators. We also introduce a new modulation parameter associated with the additional eigenfunction in the kernel of the operator $L_{I}$, which enables us to perform the modulation analysis and establish the uniqueness of exponentially decaying solutions to the linearized equation.