I made the mistake of not sanding the round muffler of PZ IV before I glued it on.I need to remove off the back of the tank to sand it.How do I remove it glued on with Tamiya cement?Thanks.

Try to moisten attachment points of muffler stubs from the inside of the hull rear with Tamiya extra thin or (better) with Mr.Cement S. Use your cement sparingly. Place a drop of glue, then pull gently, it should work.

Modelling glue melts the plastic creating a weld joint, so there is not really a way to "unglue".
If the joint point is small you can do as Paul has suggested and it may work, otherwise you have to make a clean cut and glue again.