The hypercube isn't 4-dimensional
It's actually 6-dimensional: it has 4 dimensions of space and 2 of time. But before I explain why it is 6-dimensional, I'll explain why it isn't 4-dimensional.
Most people would consider any normal old cube to be 3-dimensional, but it is in fact 4-dimensional, since it occupies the 3 dimensions of space and the one dimension of time. If it didn't occupy the 4 dimensions, the "normal" cube would literally only exist for an instant. (just as a 2-dimensional square has infinitessimal depth and only exists at a single point in the third dimension)
Now, imagine we build a perfectly normal cube with a side-length of 5m at 06:00:00 and dismantle it at 06:06:59. So, this cube has dimensions of 5mx5mx5mx419s. Now put someone in that cube. If that person exits the cube, they just exit into whatever place the cube was built in (analagous to exiting a house into your neighborhood, or exiting your car into the parking lot or driveway). If you were to attach other cubes to the sides of the first one to create a network of cubes, it would be no different from the cubes in the first movie (Note, however, that it is implied that the entire hypercube consits of only a single room at different points in time). Also, if you were to just wait until 06:06:59, when the cube is supposed to be dismantled, the cube is simply dismantled around you and you are left in whatever place the cube was built in the first place (like having your house being dismantled around you to be left in your yard, or your car dismantled around you to be left in your driveway). Obviously, such a cube is no different from the boring old cube from the first movie.
So, how many dimensions do you need to get a cube like the one in the movie? The answer is 6, and the cube isn't really a "hypercube" either... it's closer to a "hypertoroid". I'll explain why: In the movie, as I have said, it is implied that the entire cube is only one room. So, when a person leaves through a side of the cube, they simply appear on the other side (like we saw near the end of the movie, though earlier in the movie, they would actually appear back in the cube at a different point in time). Instead of starting with a cube and try to figure out how we would accomplish this, let's start out with a line. We want to deform the line so that by going in one direction, we'll eventually end up back where we started. How do we do this? By turning the line into a circle. So now we have a 2-d object, which locally appears to be 1-dimensional (since one can still only move back and forth along the "line".) Now, let's say we want to do the same thing with a square, so that if one moves off one edge, he reappears on the opposite side, or if he walks in one direction, he'll end up back where he started. We do this by mapping the square onto the 3-dimensional shape called a torus (a doughnut or bagel shape). The reason we use a torus instead of a sphere is the sphere would only truly wrap in one direction (for example, on earth, if you were to just keep going north, you wouldn't suddenly appear at the south pole after reaching the north pole, instead you start coming back down on the other side of the earth.) Also note that while the torus is 3-dimensional, it still locally appears to be 2-dimensional, since we are only walking on its surface. Now all that we do is apply the same mapping of a cube onto a 4-dimensional hypertoroid, and we get a shape which is locally 3-dimensional, but wraps in all directions - just like the hypercube in the movie.
So we now have a shape which has 4 spatial dimensions and 1 time dimension, for a total of 5 dimensions. But, with only one dimension of time, it still flows perfectly normally. This is simply solved by adding another dimension of time, but which is orthogonal to the other time dimension and the 4 spatial ones at inside of the cube, but not at the edges. (orthogonal means "at a 90 degree angle". If 2 axes or dimensions are orthogonal, then motion in one does not correspond to motion in the other, ie if I move forward, then I haven't also moved left or right. If they are not orthogonal, then motion in one *does* correspond to motion in the other.) In this way, we can have the time in one axis pass normally, while time in the other axis passes as we move through the doors in the cube. This even allows it to seem as if passing through a door moves you into a completely different room, since now you're in a different point in time in one of the axes, and your timeline doesn't intersect with the original timeline (until the cube collapses at the end, that is). Another effect of this (though it would take too long to explain why) is that people moving in the same direction through time would experience time passing at the same rate, but 2 people moving in different directions would both appear to one-another to move more slowly (seemingly [but not] paradoxically, and also contrary to the movie's portrayal of one appearing to move more slowly while the other appears to move faster. Thus, many effects in the movie can be explained by adding this second time dimension.
So, the hypercube isn't 4-dimensional: a 4-dimensional cube is nothing special. In fact, it is a 6-dimensional toroid.