Zbrush 4 bug....
Well would you believe it, I finally got the time to start getting into Z4 at long last as its been months since I last played with it, so I was eager to get a model sculpted up....
anyway on loading a 3.5 ztl file, Z4 threw up an error about an invalid mesh type and said the old shadowbox mesh will be replaced with new one, then updated my model with a nice shadowbox plane and a set of eyeballs from the actual model!! Pain in the arse.
Anyway for those of you who may also have this problem, I did little digging and Pixologic are aware and are on their way to fixing it.
The workaround though for now, is to open your ztl file in the earlier version of zb, switch to the lowest level and export an OBJ, then switch to the highest level with all the lovely detail and export it as an OBJ too, also make a note of the highest subdivision level of the model.
Once in zbrush4 import the lowest/base mesh OBJ and divide it to the highest subdivision level as mentioned previously. Then import the high res OBJ mesh. This will over write the lowlevel mesh but will retain all the subd levels down to the base giving you total control over the mesh and details at various levels. Then you can save out a new Zbrush4 tool.
A pain in the butt, but it is a good solution until it gets fixed