As you might expect, there are a lot more ways to rotate in 4D than in 3D, which presents a problem in designing sensible & ergonomic video game controls.
How Rotation WorksBefore getting to how a player might control their 4D orientation, it will be useful to go over some basics of how rotation actually works. In three dimensions, we are used to thinking of rotations as occurring about an axis. That, however, is a simplification unique to 3D space. In the generic n-dimensional case, rotations are defined by a set of planes.
In one dimension, there are no planes, so rotation is impossible. In two dimensions, there is exactly one plane, and rotations in that plane can be defined simply by their magnitude left or right. In 3 dimensions, however, you suddenly have 3 possibilities for basic planes of rotation, and an infinite number of linear combinations thereof. Specifically, one can rotate in the XY plane, the XZ plane, and the YZ plane. Whichever plane you choose, there is one axis left over, and the unique one-to-one pairing of axes and their perpendicular planes in three dimensions is what lets us describe rotations with axes. In 4D, however, there are six basic planes (XY, XZ, XW, YZ, YW, and ZW), and there is a whole additional plane perpendicular to any chosen plane of rotation- not a single unique axis. The simplest way of describing a rotation in 4 dimensions thus involves directly specifying the plane in which it occurs,
(Incidentally, the fact that any chosen plane is perpendicular to an entire additional plane means that 4D objects can have two independent rates of rotation in perpendicular planes, something not possible in 3D. But, that's a complication I can safely ignore for maze-navigating purposes.)
Straightforward Control SchemesIn 3D, we can often just assign one key to each direction of rotation in each basic plane; e.g., using WASD+QE for pitch, yaw, and roll. With six basic planes, however, that would require 12 keys. Now, that's doable; we could, for example, mirror the WASDQE arrangement on the other side of the keyboard, assigning the XW, YW, and ZW planes to UIOJKL. As far as I can tell, this is the maximally flexible scheme, allowing for linear combinations of any set of basic rotations.
But, we can cut down the number of keys used to just six again by exploiting the nature of 4D rotations: with one key for each of the four coordinate axes, and two keys for positive and negative rotation, the player can directly select which plane they want to rotate in. This (with some modifications to allow traditional controls for purely 3D motion) was the original scheme I implemented for my game.
The downside of the simple plane-selection scheme, however, is that it requires simultaneously pressing three keys to perform any rotation- two axis keys to define the plane, and a sign key for which way to rotate in that plane.
Alternatives for Reducing the Number of Keys
Another option is simply to assign one key to each of the six basic planes, again with two keys to determine the sign of the rotation. That's a total of 8 keys, requiring two key presses for each rotation. I worry, however, that remembering the key mappings for each plane might get annoying.
We can improve things a little more by shifting some of the responsibility for selecting planes onto the sign keys. If we have two sets of sign keys (say, the up/down and left/right arrow keys), then we only need half as many plane keys; a single key selects one plane to be controlled by the up/down arrows (or whatever else you pick for the sign keys), and another (preferably perpendicular) plane to be controlled by the left/right keys. That's a total of 7 keys, again requiring two simultaneous key presses for any rotation, and regaining some of the ability to compose basic rotations. If you choose a default set of planes for the sign keys, however, then you need only six keys total, as in the straightforward plane-selection scheme, but only 2/3 of all rotations require holding multiple keys, and you get the ability to compose some rotations.
In order to reduce a player's learning curve, it would be nice to ensure that normal, purely 3D rotations still use traditional controls. It turns out there's a very simple way to achieve that, extending the same logic used above. If we have six sign keys, and and a default selection of planes, then only one plane-selection key is required to switch the default three rotation planes out for the other three rotation planes. It is then trivial to choose the default planes to match 3D pitch, roll, and yaw. All 3D rotations then require only a single keypress, with an average of 1.5 simultaneous keys required for all rotations.
The current scheme I have implemented for my game is a slight expansion on this idea, with 3 plane-selection keys (for a total of 9 keys) that map the sign keys (implemented with WASDQE) to different sets of planes. The idea behind this design is to make it possible to create rotations in combinations of any two planes simultaneously, without requiring the full 12-key arrangement. Whether this is the best approach, however, I am still uncertain. It may be that, at least for this kind of game, there really isn't any great need for the ability to compose rotations in multiple basic planes, in which case the 7-key hybrid scheme may be better. On the other hand, if that is a desirable property, it might be best to simply go all-out with the full 12-key scheme.
Conclusion: More experimentation is required!
Next: Implementing 4D Rotation