difference of physical size of the two is considerable!
Width is about same and both take 43mm filters but the f4 is tiny, very short, while the f2.8 is surprisingly long. the lenght of the f4 is 30mm, of the f2.8 52mm.
check "Baulaenge" in this German site: http://olypedia.de/E_Zuiko_Auto-W_4/25
it says both weigh 160 grms, that must be a mistake, correct must be that the f4 weighs 120 grms http://fotonode.com/db_photo/content...auto-w-25mm-f4
and the f2.8 160 grms http://fotonode.com/db_photo/content...uto-w-25mm-f28
in comparison your f1.8/38mm is 35mm long and weighs 135 grms
if needed, tomorrow I can take a photo of them together. for now see this:
Pen-F lenses + NEX5n
, on Flickr
the f2.8/25 left back row, the f4/25 second from left front row. ( the f1.8/38 mounted on the camera. I changed them to Leica M, that added 1.15mm to each )
( interestingly the f2.8 is that long that it even is longer than the Olympus OM f2.8/24mm for "FF" both having a Sony E mount adapter mounted! The Pen-F's diameter is smaller.
Of course the Sony E adapter for Pen-F lenses is much shorter than that for OM, but the Pen-F lens that much longer. given the OM 2.8/24 must be one of the smallest 24mm lens for 35mm )