I would not use five different types unless this hedge is extremely long. How long is it?

Yes, length is an important consideration! If a suitably long enough length, using a mix of different shrubs is not really an issue.....just kind of a scaled down version of a tapestry hedge/hedgerow, which is a common enough landscaping concept. But I am more concerned about the height and spacing. Few shrubs will grow to your desired height and then stop so you will need to be concerned about pruning frequency and how that might affect the growth habit and flowering of the shrubs you select. And the spacing of 12-18 inches is very tight - more in line for how small groundcovers are set out rather than full size shrubs. I'd visit a local garden center and see what dwarf selections are available that may appeal to you and suit your climate. And these will all need to be spaced at half their mature spread, not at some arbitrary measurement :-))

Judging by the information on the OP's thread in Shrubs, the site for the hedge seems to be only about 6 feet long, unless I've completely misunderstood. I've tried to address the impracticality of the concept over there.OP's other thread

Yeah, I posted over
there too after an initial drought of responses here. Anyway, all responses are
unanimous in that all plants ought to be of the same kind. My cheapest
(actually gratuitous) option would be a Goldenrod hedge. These self-seeded
plants grow in dense clumps up to 3' to 4' tall in my backyard. It should be
possible to transplant and re-arrange them to form the hedge, anyone here
actually tried that?

A
perhaps tidier option is a short conifer hedge, like with the common Thuja
occidentalis 'Emerald Green'. Around here, they
are always pruned to 5' or taller, but I wonder if they can be pruned to 3' or
4' if planted small and dense at 12" to 18" centers?

I don't think Emerald Green can be reasonably maintained that small. I'd look for a smaller cultivar since their natural growth is over 10' tall by 3'-4' wide. You'd end up with a lot of bare wood trying to maintain them that small. Look for smaller cultivars.

Have not been following the discussion on the other, similar thread but agree that a 6' length is far too small to attempt a mix of shrubs. And you wouldn't need more than one or two to completely fill that length as well!! Unless you are going with something like boxwood, which is a popular selection for a short (height) hedge. For your zone, I'd stick with the very hardy Korean box. I am not all that familiar with short flowering shrubs hardy to a USDA zone 4 but alpine currant is extremely cold tolerant and can be sheared into a hedge or pruned to keep height in check.

Since the discussion
has moved on to the single type hedge, the title of this thread has become
obsolete (and misleading) I have opened a new thread for the conifer hedge here

http://forums.gardenweb.com/discussions/4220105/conifer-for-a-low-hedge

I may open a new thread for the Goldenrod hedge in the 'Perennials' forum. This
site does not appear to have a 'Wildflower' forum, it has a 'Wildlife Garden'
forum, but that is something else.

Thanks for the
comments!

Thread for Goldenrod
hedge http://forums2.gardenweb.com/discussions/4220275/m=23/goldenrod-hedge

Thanks again for the
comments!