Short par 4's, done well, are tough to beat. I really look forward to them when playing on familiar courses and find them probably the easiest to remember after playing a new course.
What's too much of a good thing? Thinking inside the box, if a typical course has about ten par 4's, my gut tells me that much over a third of those holes is pushing it, so three is OK but four is pushing it. As Tom Doak points out, it's is a matter of variety. On an undulating site with variable day to day winds from different directions, more could work well.
A short par 4 doesn't have to be reachable to be interesting as Brent Hutto points out. It helps. A par 4 that always takes the driver out of consideration is weaker than a hole that invites a choice.
As to risk, there are two parts to the equation: how likely is "some bad thing" to happen and what is the effect if "the bad thing" happens. On the best short par 4's, I think the likelihood of a bad thing happening for those going the extra yard off the tee or taking the aggressive line ought to be high but it's not at all necessary to ensure the bad thing be catastrophic.
I absolutely respect short par 4's, every bit as much as long par 3's, long par 4's, short par 5's: they're all swing holes.