Sorry guys but why all this enphasise on preshading, Panzer Grey is already a damn dark colour and if one is using preshading to create demarkation in the panels I rather suggest controlled pin washing.
Personally I would more suggest an overhead overall gradation colour, to highlight the modular block design of the tank. Overhead spraying as Alex suggested will give a 3D dimension to the tank, hence the turret would get a more highlighted area. Panzer Grey is a very tricky colour, apart that till date no one is 100% sure what was the tonality [please Marco do not start any thread with suggestions LOL]it is a dark mute colour and needs some highlighting to create definition IMO.
Tiny I think you have already have a very good base, I would try to mix the Drak Grey that you have with a lighter greyish blue mix to create a highlight and overhead spray the tank from a particular angle, work in diluted sessions and progress to the highlight in stages. NB. I am still not convinced with that gun mantled, I see it too long for an early Pz4, might I be wrong as I am more in love with ruskie tanks ... check it out with referances mate ...
Keep the good work, you have acheived a very smooth finish coat ... PROSIT!