I think one excellent 4d printing project that (to my knowledge) nobody has done yet would be to print the Optiverse. This is John Sullivan's minimal elastic bending energy version of the $\mathbb RP^2$-midpoint Shapiro-Morin sphere eversion.
http://www.math.uiuc.edu/~jms/Papers/isama/eversions.pdf
This would take quite a bit of work as you would have to make some fairly intelligent choices of how to make cuts in the surface in order to see what's going on, yet not lose track that this is a sphere.
But who knows, maybe Sullivan has a version of this on his office desk? It wouldn't surprise me.