added exercise4.rst

docs/exercise4.rst

+Exercise 4: Extending a workflow
+================================
+
+Workflows in FireWorks can be extended at any time of their life cycle, 
+particularly in states *READY*, *RUNNING* and *COMPLETED*. If the appended 
+firework or workflow is appended as a detour then the child fireworks may not be 
+be *RUNNING* or *COMPLETED*.
+
+
+Problem 4.1
+-----------
+
+Using the extend the workflow from the solution of **Problem 3.1** 
+(**image_reconstruct.json**) with the workflow from **Problem 3.2** 
+(**image_swirl.json**) to reuse the image produced by the former as input in the 
+latter workflow. For this, first copy the workflow **image_swirl.json** from 
+**exercises/solutions/3_files_and_commands** and modify it so that the input 
+``original image`` in all fireworks is passed from a parent firework::
+
+    "original image": {
+        "source": "montaged image"
+    },
+
+Also make all firework IDs negative integers (required by the appendoing method).
+
+Then identify the firework ID of the firework providing the final image as output::
+
+    lpad get_wflows -n "Image reconstruction" -d more
+
+Finally, use the command::
+
+    lpad append_wflow -i <ID> image_swirl.json
+