Lo Siento, a Barcelona-based design collective has created a set of 4D letters, readable from all sides. Created from simple  white cardstock, the designers describe the project thusly:

4D Typography is the result of the intersection in an orthogonal way in space of two extrusions of the same character, which allows the spectator to read it from two different positions in space at least.

An observer searching to enjoy a particular architecture is forced to move around and through it. The change in perspective generates new spaces in which light acts in different ways. In this case, it is the typography that makes the effort of abandoning its two dimensions to approach the architectural sense. It does not resign with a third dimension; a fourth one is necessary to complete the reading possibilities. By hanging the typography, the reader is allowed to surround the characters in order to understand all their shapes.