Bad timing for a 4GB card imho.
If it were 8GB or even 6GB, I think they could have a big massive hit. But 4GB @ $849, that's not going to move very well (to be fair Titans are small market too, correct?)
Anyhow, my personal theory with absolutely zero hard evidence is that HBM is a bitch to manufacture, so supplies will be limited. Result is trying to dress it up as exotic and high per unit price as possible while the tech really matures.
An AMD buyer would be far better off with a pair of 8GB 290s over an $849 4GB 390X, or is that off base? I mean obviously for 4K/Surround style setups. Obviously a single 1080 display should largely be fine @ 4GB, but who spends $849 for a 1080P single screen GPU?