If a line is first dmension and a square is 2D, and a cube is 3D, how wud u describe a 4D?

As we are three dimensional creatures it is difficult for us to imagine four dimensional objects as it is difficult for an ant to imagine three dimensional cube or sphere. For it these are two dimensional surfaces only. But we can venture to draw a four dimensional cube on paper as we draw three dimensional cube on two dimensional paper. We make two parallel parallelograms and connect their corresponding corners to get a perspective view of a cube in two dimensions. Here the incline faces of cube are shown in te same plane. So we can draw two such cubes side by side and connect the corresponding eight corners to get a diagram of a hyper cube in four dimensions. Remember that the faces of such a hyper cube will be three dimensional, as the faces of a cube are two dimensional.

A fourth dimension isn't neccesary a dimension in space, in the accepted model time is the 4th dimension. So 3 dimensions of space and 1 dimension of time are the ones we can experience but there are theory's ( string theory ) that say there are much more dimensions.

Read up general relativity einsteins there describes our universe as a fabric of spacetime where time is related to space.

the 4th dimension is time. imagine a cube as it was, as it is now, and as it will be all at once. that's the 4th dimension. because we experience the world in 3 dimensions, it's difficult to picture any others

A hologram.

4D "cube" is the hypercube or tesseract