I've been working on a Poser project for awhile, storyboarding it. I had in mind several face morphs to use for V4. and a uniform, which they all would wear, but it's for A4.

I had not realized it before, but V4 can't easily fit into A4 clothes (like I can no longer fit into my teenage jeans T.T) and A4 cannot accept V4 face morphs.

So really my question is, is this statement true?  V4 and A4 share the same body mesh, and although they also can use the same face textures their polygons are different for their heads, which prohibits the use of the same facial morphs?

nope not true at all, A4 is a morph set for V4 so as long as the V4 clothing has A4 morphs you can go between them quite easily(as far as I know there is no clothing for A4 that isnt at base V4 clothing, that just wouldnt make sense). as far as facial morphs V4 morphs will work on A4 but due to the fact that its adding one morph ontop of another you have to be careful how high you set the other morphs before you get distortion.

All of my A4 clothing fits my V4 character but only because it's like wdupre stated. You have to have the A4 morphs to inject into V4.  You have to have both bases and the Victoria Morphs ++.  If you want to use the V4 facial expressions then try reducing the Aiko 4 morph dial to 0 . This will give her a V4 face and the expressions should work then. 

I did a morphing workshop where I showed that the 2 bases were exactly the same (after I inject the morphs).  The V4 facial morphs should distort the closer you get to the Aiko 4 look.

Jerr3d.. sounds like perfect timing for you.. There's nothing like finding a solution soon after asking the question.

Ah yes that is an error with Poser's External binary morph system, it doesnt work well with some figures, causing problems when you try to open a character you have saved to your library, my recommendation is to turn save external binary morphs off in your preferences and you wont get that warning in the future.