Not long ago, Cirici and Wilson defined a Dolbeault cohomology on almost complex manifolds to answer Hirzebruch's problem. In this paper, we define a refined Dolbeault cohomology on almost complex manifolds. We show that the condition $\tilde h^{ 1,0}=\tilde h^{ 0,1}$ implies a symplectic structure on a compact almost complex $4$ manifold, where $\tilde h^{ 1,0}$ and $\tilde h^{ 0,1}$ are the dimensions of the refined Dolbeault cohomology groups with bi-degrees $(1,0)$ and $(0,1)$ respectively. Combining the partial answer to Donaldson's tameness conjecture, we offer a sufficient condition for a compact almost complex $4$ manifold to become an almost K\"ahler one. Moreover, we prove that the condition $\tilde{ h}^{ 1,0}=\tilde h^{ 0,1}$ is equivalent to the generalized $\partial\bar\partial$-lemma. This can be regarded as an analogue of the Kodaira's conjecture on almost complex $4$ manifolds. As an application, we show that the Kodaira-Thurston manifold satisfies the $\partial\bar\partial$-lemma. Meanwhile, we show that the Fr\"olicher-type equality does not hold on a general almost complex $4$ manifold, which is different to the case of compact complex surfaces.
Categories: math.DG math.GT math.SG