On further examination, that tiny little tripod collar placed back at the aft end of the lens wouldn't provide very acceptable balance for a 400/4 either. If it isn't the already existing 300/4 then the only other possibly option is this is a new 400/5.6. Front element could well be big enough for a 5.6 lens. Tiny collars aren't unusual on consumer-level 400/5.6 lenses either. Yep, either it's the "old" 300 or a new 400/5.6. More of a consumer lens than a pro lens if true. Price on a 400/5.6 would be lots closer to the original prices Pentax often seeks to hit on introductions--
DSLR's, 200/2.8 and 300/4 all tended to hit the market around $1200 U.S. which is about where a 400/5.6 would price out--give or take a couple hundred. So if it's a new product, I'm betting it's a 400/5.6 that will be introduced around the $1200 U.S. price point.
Bad news for all of us hoping for a "real" big gun. I had hoped the announced discontinuation of the FA*600/4 was actually a foreshadowing of a new DA* version of that lens. The F* and FA* versions of the 600 have very few spots needing weather sealing. It would be an easy DA* conversion with just a few gaskets and seals plus the DA* finish. Oh well, won't hold my breath either way.