Immersed isometrically in $\mathbb{R}^4$

Question

I know well that there is an isometric immersion of the hyperbolic plane in $\mathbb R^5$ due to Rozendorn and that the hyperbolic plane cannot be immersed isometrically in $\mathbb R^3$ due to Hilbert, also I think that the problem in $\mathbb R^4$ is still open. So questions arise that I can't answer:

1. Are there surfaces with negative Gaussian curvature immersed isometrically in $\mathbb R^4$?
2. Are there surfaces with negative Gaussian curvature with $K\leq const<0$ immersed isometrically in $\mathbb R^4$?
3. Are there surfaces with constant negative Gaussian curvature immersed isometrically in $\mathbb R^4$?

Does anyone know where I can read more about the existence of complete surfaces with constant Gaussian curvature -1 immersed isometrically in $\mathbb R^4$?