Can anyone give me an idea of the ease/distance for transferring from U4 or U5 to U6 at Odeonplatz
It's super easy. A completely underground transfer within the station complex so no need to walk across a street or up to the surface.
Exit U4 or U5 at Odeonsplatz, follow the signage at the walls that point to U3 and U6 which share a platform on an upper underground level. Walk the underground passageway, then take the escalator up. You'll arrive directly on the platform of U3 and U6. All in all this will take 3 to 5 minutes usually.