DAZ sells the Texture Convertor 2.  It has a plugin that will convert V4 textures for use on V3 and V3 textures for use on V4.

Note: the app does not come with the plugins unless you buy the bundle.  If you just buy the app, it won't really do anything so read the product page carefully.

The V4 to V3 figure accepts V4 head morphs and uses V4 textures.  The body has V3 rigging and body shape so you can use your V3 clothing out of the box.  The body does not accept the V3 body morphs.

If you are having trouble with the V4 to V3 figure, make sure you run the bat file for the V4 to V3 figure found in the !DAZ folder inside the library folder of wherever you installed V4.