For $1<c<c_0$, for some constant $c_0$ the circle method will yield an asymptotic of size $$E(A)\sim \mathfrak{S}N^{4/c-1.}$$ where $\mathfrak{S}$ is a constant. In fact, when $c$ is not too large, one can even obtain an asymptotic for the additive energy of the Piatetski-Shapiro primes (see [Balog and Friedlander][1]). Letting $S_{A}(\theta)=\mathbb{E}_{n\leq N}1_{A}(n)e(n\theta)$ we may write $$E(A)=N^{3}\int_{0}^{1}|S_{A}(\theta)|^{4}d\theta,$$ or alternatively $$E(A)=\frac{1}{N}\int_{0}^{1}\left|\sum_{n\leq N^{1/c}}e(\lfloor n^{c}\rfloor\theta)\right|^{4}d\theta.$$

**Remark:** I am not sure how large $c_0$ can be, but the asymptotic definitely breaks down for $c=2$.


  [1]: http://msp.org/pjm/1992/156-1/pjm-v156-n1-p03-p.pdf