If you are not required to use the same gear as the photographer you're studying, then you don't necesssarily need the 4x5. It would be better, as previously mentioned, to study the lighting technique - when it comes to portraiture, lighting is often more important than the camera that is used. I think, however, that Karsh used the 4x5 largely in neutral position, though.
Considering the "neutral position," however, and using a 4x5, perhaps a field 4x5 might be an option. I don't know if this would be suitable for you, though.
As for getting better detail without the size of a 4x5, maybe the camera to be used should be a Mamiya 7 rangefinder - or if you can find them, Fuji made some medium format rangefinder cameras a number of years back, but unfortunately, their model numbers escape me at the moment.
Good luck with your endeavours - and I would like to add that what you're doing and what you've accomplished is very admirable, indeed.