More of either size would be better than just 4 x ¾”. If you are going to the trouble of building a 4” shotgun condenser you want to be certain you can get full reflux out of it.
It all has to do with surface area and volume. Higher surface area to volume ratio will give you more cooling, the same volume of ½” tube will have much more surface area than ¾” tube (there will just be more of them).
Don’t worry about how much water is in the condenser, water has excellent heat exchanging properties (as does copper) so you don’t actually need a massive volume in there, just good flow.
You need to consider your cooling water temperature also, if you are from the north and are using an IBC or similar your cooling capacity is going to be limited.
You may not want to go the the lengths of this
http://aussiedistiller.com.au/viewtopic.php?t=11458#p199048 but I strongly recommend going a bit further than you propose.
I also suggest a lot more research about the type of still you are intending to build before you go much further. There is a lot of work in one of these things, you’d hate for it not to work the way you intend it to :handgestures-thumbupleft:
RC is Reflux Condenser
PC is Product Condenser
Cheers,