I own LR 3 (full) and LR 4 (upgrade). Will it be possible to upgrade the software from LR 4 to LR 6 directly?
In the past, the upgrade version of Lr upgraded all versions from v1 onwards, and I see no reason why this should not be true this time, but can make no promises, mainly because I have no connection to Adobe.
So yes, you will probably be able to do as you want.
Adobe has always allowed upgrades from any earlier version to the newest. (i.e from 1,2,3,or 4 direct to 5)
So I think it would be a safe bet that you will be able to upgrade from 4 to 6. Adobe has not yet made any announcement.