It may sound crazy but, in this 3D world there is nothing 2D, other then our shadow. Is it good to say that the shadow of 3D object is 2D, then shadow of 2D object would be 1D. Hence this 3D universe is shadow of 4D world?
Depending on what you think a shadow is, probably not.
If you think a shadow is a boundary, then no. The boundary of a boundary is null. It's not one dimension less, it's the empty set.
If you think a shadow is a projection, then also no. Projections have the property that if you perform the projection again, then you get back the original projection. That is, if P is the projection operator, then P * P = P. Or, to take a specific example: If your chair is 45 cm in the "x" direction, then length of the "x direction" of your chair is also 45 cm.
The only possible way it could make sense is as a cross section. The intersection of a cone with a plane is circle, ellipse, parabola, or hyperbola. If you "fill in" this figure you get a segment of a plane. Then the intersection of this with a line is a line segment.
The problem with that is, it presumes there is already a plane to intersect the cone. You need to define both the plane and the cone apart from their intersection.
So, supposing there is a 4D universe that we are a cross section of is, so far, not helpful. You need to define this 3D universe, and the 4D universe, both independently. And once that is done you need to define the intersection that gives the cross section. And, so far, you have done none of that.
So, no, what you are saying isn't helpful.
The Universe cannot be regarded as 3D as it is factually wrong. To imply that a time slice now is like a picture in 3D is equivalent to saying that there time is experienced in between quantum jumps of planck time ticks. Makes absolutely no sense, because time isn't a concept at the planck scale. After planck time, the Universe changes quantum states. That's about it. Time at higher levels is emergent from this activity of change of quantum states in planck time. Between two states, there is no now. This misconception, if abolished, clears the air and we now see that what you call now, is not there. There is no "planck frame" which is like frozen in time.
The idea is persistent because of the block universe model. In that model it is supposed that for an observer, the block universe is like a loaf of bread, but this bread, isn't cuboidal. Rather it progressively skews as you'd like to make a slice of "now". This is because there is no universal now that all observers agree to. So a distant universe, will not agree on the simultaneity of events, and subsequently, of "now". Hence you are left with your unique perspective.