Notation issue regarding differential equations

I am given the following problem:

Find a basis of solutions for the equation: $u^{iv} + 2u'' + 3u = 0$

The notation is an exact duplicaticate of what our professor used in his notes. Does anybody know what $iv$ stands for here? We are given no further information about anything else.

-
Just as an aside, unless your professor absolutely insists on it, my personal suggestion is to never, ever use this notation. –  Christopher A. Wong Jan 28 '13 at 23:47
Agreed. When in doubt, use Leibniz's $\frac{\partial^n u}{\partial t^n}$ notation. –  Emil Lundberg Jan 29 '13 at 0:05
I agree with Christopher and Emil. This notation is awful. A while ago I asked this related question. –  user26872 Jan 29 '13 at 4:17
What makes it even worse, is that we have now began the study of 2nd order differential equations with constant coefficients. In this context, one may expect the appearance of material from complex variables, so I was afraid that the $i$ in the above heinous notation was in fact the imaginary unit. –  user44069 Jan 29 '13 at 5:43

$iv$ is Roman numerals for $4$. He means the fourth derivative $u''''$.
I was thinking the same thing, but they usually denote it by $u^{(4)}$. In any event, thank you very much! –  user44069 Jan 28 '13 at 23:36