In UE4.3, this particular function is missing the setting to specify the material target. This is easily worked around by placing the following mesh at 0,0,0 in the world, and setting the Scale to match the number specified in "Sheet Size" on the Blueprint (Defaults to 1000):
/Engine/ArtTools/RenderToTexture/Meshes/S_1_Unit_Plane.S_1_Unit_Plane
Then simply apply the desired material and it will work.
You must enable the ability to export buffer visualization targets in the editor. In the upper-left corner of the Perspective viewport, click the Down-Arrow button to open the Viewport Options menu, select High Resolution Screenshot, then enable Include Buffer Visualization Targets.
Leave this window open! This window will move behind the editor and should not interfere. If you accidentally close the window, the buffer images will not render.
Rendering the Textures
Select the desired buffer targets under Capture Settings; Make sure the High resolution screenshot window is open and the Include buffer visualization option is enabled; Play the game in a New Window, bring up the console using the ~ key and type "ke * rendertextures" at the console and hit enter.
You should get a popup message indicating that images were saved, the message is actually a clickable link to the folder location. If you are not able to click the link in time, you can find the saved textures here:
\YourProject\Saved\Screenshots\Windows\