With perfect gear distribution spread and coordination you could probably get up to about 3.8x. However we're talking bots playing with perfect information about all gear that drops. The additional time it takes to coordinate the drops, make the trades and organize the builds in general eats up a not insignificant amount of time. Then you each get different prophecies and won't always be completing the same content for your prophecies. So with very veteran players, I'll give you 3x at best. Average use case is probably 2x on a good day. I don't really have any data to back that up, I just think that the 4x suggestion is wishful. We can then balance the group find vs solo with a little more granularity by adjusting the difficulty scaling per additional party member. We aren't aiming for a 1:1 here though, a full party that is planned out well with people who work together should probably end up a little better than a solo player. Uncoordinated groups will probably be a little worse off than solo players.
We aren't really aiming for perfect balance anyways. It's a waste of time to get it just right. As you already mentioned, with leaderboards split, it shouldn't really matter eh?