First, let's use less ambiguous terminology so that we don't confuse ourselves over what is being said. The term "3D knot" is ambiguous because there are two things going on here: the dimension of the ambient space (3D), and the dimension of the extensions of the thing being knotted (e.g., a rope is 1D as far as knots are concerned).
So I'm assuming you mean that in 3D (ambient space), a 1D rope can be tied into a knot, but you're wondering why a 1D rope cannot be knotted in 4D?
The easiest way to understand why this is not possible is to imagine that we take a 1D length of rope in 4D and squeeze it between two parallel hyperplanes, so that it essentially only has 3 degrees of freedom. Then it is essentially equivalent to a 1D rope in 3D, so obviously we can tie a knot with it.
Now, the reason the rope is knotted is because at one or more points along its length, it would need to "cross over" another part of itself in order to become untied. Just think of your regular knot in 3D: if a knotted piece of rope can pass through itself at certain critical points, then the knot can be undone without actually untying the knot. Let's call these points "crossing points". The knot is a knot only because the rope can't pass through itself at these crossing points: another part of it is blocking itself, and that's what holds the knot together.
Now suppose that after tying the knot, we remove the confining parallel hyperplanes, so that the rope again has 4 degrees of freedom. Take one of the knot's crossing points. Notice that the part of the rope that's blocking itself lies only in the 3D hyperplane that it was originally confined to. But now that the confinement is removed, the blocking part of the rope can simply be pulled in the 4th direction so that it no longer blocks the movement of the blocked part of the rope. Thus, we can simply pull the blocked part of the rope over to the other side, and thus the knot becomes undone. Intuitively speaking, this means that any knotted 1D rope in 4D can be unknotted simply by pulling its ends: the knot will just untie itself -- it is actually no knot at all.
You may say, well, what if we knot the rope such that the blocked part is itself blocked by another part so that we can't simply move it out of the way in the 4th direction? It turns out that no matter how you try to tie the knot, a 1D piece of rope simply does not occupy enough space in 4D to be able to block every possible movement of other parts of itself. Mathematically speaking, a knotted 1D rope in 4D is topologically equivalent to the "unknot" (i.e., no knot at all). No matter what you do, there's always some direction in which we can pull the rope so that the crossing point becomes undone. The reason for this is that a 1D piece of rope is not enough to block that extra degree of freedom in 4D; there's always a leftover degree of freedom that lets you undo the knot trivially.
The only way a knot can hold together in 4D is if the "rope" is extended not only in 1D, but in 2D. That is, in 4D, the only way you can knot something is to use 2D sheets. One example of a 4D knot is the Klein bottle -- which is actually a misnomer, because it doesn't hold any water in 4D. It's essentially a "knotted sphere", a sphere knotted with itself in such a way that there's no way you can pull it apart back into a "normal" sphere.
And if you think this is very strange, you're right, but wait till you see 5D. In 5D, even 2D sheets won't knot. Instead, you have knotted 3D realms (knotted space, anyone?
). In general, to make a knot in N space, the object being knotted must extend in (n-2) dimensions.