Though much work has been done with respect to the existence of extremals of the critical first order Trudinger-Moser inequalities in W 1,n (R n ) and higher order Adams inequalities on finite domain Ω ⊂ R n , whether there exists an extremal function for the critical higher order Adams inequalities on the entire space R n still remains open. The current paper represents the first attempt in this direction. The classical blowup procedure cannot apply to solving the existence of critical Adams type inequality because of the absence of the Pólya-Szegö type inequality. In this paper, we develop some new ideas and approaches based on a sharp Fourier rearrangement principle (see [24]), sharp constants of the higher-order Gagliardo-Nirenberg inequalities and optimal poly-harmonic truncations to study the existence and nonexistence of the maximizers for the Adams inequalities in R 4 of the formwhere α ∈ (−∞, 32π 2 ). We establish the existence of the threshold α * , where α * ≥ (32π 2 ) 2 B2 2 and B 2 ≥ 1 24π 2 , such that S (α) is attained if 32π 2 − α < α * , and is not attained if 32π 2 − α > α * . This phenomena has not been observed before even in the case of first order Trudinger-Moser inequality. Therefore, we also establish the existence and non-existence of an extremal function for the Trudinger-Moser inequality on R 2 . Furthermore, the symmetry of the extremal functions can also be deduced through the Fourier rearrangement principle.