  Let $\mathbb{F}_{2^m}$ be a finite field of cardinality $2^m$,
$R=\mathbb{F}_{2^m}[u]/\langle u^4\rangle)$ and $n$ is an odd positive integer.
For any $\delta,\alpha\in \mathbb{F}_{2^m}^{\times}$, ideals of the ring
$R[x]/\langle x^{2n}-(\delta+\alpha u^2)\rangle$ are identified as
$(\delta+\alpha u^2)$-constacyclic codes of length $2n$ over $R$. In this
paper, an explicit representation and enumeration for all distinct
$(\delta+\alpha u^2)$-constacyclic codes of length $2n$ over $R$ are presented.
