As a poster, you should be able to get a 16x20 as 2 sections of a poster, each 8x10. It does depend on the printable area of the page without any extra page margins set, is that greater than 8x10?
Assuming it is, you need to edit the page margins in Qimage (which version btw?) from the Page Formatting menu to get a printable area of exactly 8x10 and achieve your other margin restraints.
If the printable area has to less than 8x10, you will need to make a 2x2 poster, each of 4x5, and again adjust margins to get 4x5 as a printable area.
By making the printable area exactly the size of the poster section, you ensure equal sections.
Next, specify a 10x16 print size with the crop scissors on; Qimage will automatically ask if you want to spread over more pages, say yes.
I think that should do it.
BTW. don't forget we've been talking about PAGE margins here, not image borders, they should be left at zero unless you do want a border around the whole assembled poster, use B not B+ ulless you make allowances for the larger overall size.
Come back with more questions if necessary but tell us your printer model and Qimage version.
Terry

It may be a driver problem, the printer is not one I know of - it laser?
Have you printed with Qimage and that printer before, has it been ok.?
Tell us what Qimage is reporting as printable area - above the preview, what does it look like in the Preview and the full page editor?
What is the print size reported in the queue list?
One little tip, and may or may not be relevant: you may need the make the printable are a tiny fraction bigger, ie. smaller page margins, (say 0.01 ins) than the print size of the 4x5 poster section. That will make sure it does make 4 sections.
Post some screen snaps of the page margin window and the RHS of the Qimage screen, that will help us to see what you have set and we may spot an error.
Terry

Thanks.  That did cross my mind, tried it, but the printer printed it as Qimage sent, not as margins specified (might be one of those printer issues -- it's done that to other apps too)  I was able to get Qimage to split the image into pieces and spit it back out as a jpeg by using print to file.  Works for one poster, but not for a string of orders.  Also, once divided into 8x10 pieces, Qimage refuses to recognize the size as an 8x10 and keeps changing the size in the software, about 1/4 of the original.  I could do an auto-formatting run through photoshop, but it would take FOREVER and my printer wouldn't like the huge drain on internal memory.

Part of me says that I just haven't found the right combination, but it sure seems like I've tried everything I could concoct.

Thanks Ray,

I wasn't using the PCL, but I'll try installing that to see what it does.  I was able to devise a workaround, using a print to file followed by using the XP printer driver off of an older computer.  Qimage is an incredible piece of software -- I'm pretty sure I'll end up buying it.  (And, it has an active user community - thanks for your help)

After a weekend of fiddling with settings, I'm beginning to reach the conclusion it's more an issue of a buggy printer driver than anything else.  Even the edge of document colors print fine when bypassing the printer color correction.