I imagine Calc 3 = Multi D and Calc 4 = Differential Equations. Either way, it is really a matter of the individual. There are very few new concepts, if any really, that are added with each level.
Instead, Calc 2 extends 1 by building upon integration and differentiation, for me it was about as difficult but really all you are doing is using algebraic tricks for making integrals easier to solve.
Multi-D is what it says and considers the idea integration and differentiation of functions of more than one variable. Multi-d tends to be taught with the many physics examples because the mathematics is ideal for solving a great deal. Additional concepts are really just vectors.
Differential Equations is not much like the others except for that you use integration and differentiation to solve many things, however it is heavy on algebra.
In short, advancement is not really a matter of any crazy new concepts or more complex problems, many are very simple yet require more work and the chance for error increases. Setting up some problems can be difficult in multi-d once you are doing triple integrals requiring polar substitution, and problems applying Green's and Stokes theorems can get a little difficult.