No problem.
I hope that this comes to you in time. I also hope that the example you’ve stated is hypothetical - you haven’t already modeled the head, have you?
Because if you have, and detailed it through to 4 subdivs, you will run into some problems with your normal map… (remember - I’ve said that the UVs also have to be set up before you begin.)
If you don’t know much about UVs, here is a tutorial that has sufficient stuff about UVs (it is, in fact, setting up UVs for a head) IN POSER.
http://www.infinitee-designs.com/Portrait_Tutorial.htm
NOTE: I said in poser, which means that you have to either
A) Create a head in ZBrush, then export to poser (at the lowest subdiv, do NOT detail it yet), set up UVs there, export it back to ZBrush, detail it, make the displacement map, export to poser AGAIN (phew…), apply the displacement map in poser, and BAM! - You’re finally done. All this for a good model. But, potentially, a very, VERY good and detailed model, which you can then animate.
B) Instead of beginning to work right there in ZBrush, you can start working in Poser, and UV it right there, so you don’t have to export/import twice - may save you some time if you know how to model in poser well. Then, after you get your UVs in the right place, go through the rest of the process as it is described in A)
NOW, finally, about the formats… Sorry about the wordiness… Use .obj format as intermediate stage, between ZBrush and any other software (including Poser.) You see, .obj is a very universal format, kind of like .bmp, but only for vector 3D, not raster 2D. Many things understand it. This is why .obj is the only format that you can export from ZBrush (virtually no other software but ZBrush will understand the default .ZTL format - the ZBrush tools.) So, when you want to put you poser model into ZBrush, you export it to .obj, then use ZBrush Import button (Tool Pallete, more specific options in the Tool Pallete -> Import Rollout.) to get it into ZBrush. You can then save it as a tool if you’re not finished with it or as a backup (.obj won’t preserve your subdivision history and other ZBrush-specific data.) When you want to get something from ZBrush to Poser, use the reverse process - export to .obj from ZBrush (using the export button or the Tools Pallete -> Export Rollout options.); then import it into Poser.
This is a long process. But this is what most of the modelers (people who use ZBrush combined with another app) go through. This is how they made the models for gears of war (see www.unrealtechnology.com) and Unreal Tournament 3 (www.unrealtournament3.com), as well as many other cool games and animations.