Releasing any 4.5e, let alone releasing one right now, would be a monumental mistake.
WotC should try really hard to find some way to get through two more years with 4e/Essentials, and then think about releasing 5e.
4e needs cleanup. Major revisions via errata (like to save penalties, flying, mounts, etc.). Obsolete feats. Feat taxes. Math fix feats. New (better) way they do things now the product is more mature. Vast amounts of bloat. 3000 FEATS. People are talking about "fixing the warlock".
Hey, let's have a racial feat that gives a +1 feat bonus to certain saves when bloodies ... and a new feat anyone can take that gives +2 to all saves across the board.
4e would really benefit from clearing out the chaff, but with the "no .5 edition" mentality where everything is backwards compatible, it stays.
Essentials was a chance for this, but because they didn't take it you get odd artifacts like basic-attack boosting Essentials classes plus half-elfs picking up powers that count as basic attacks.
When 4e came out it was a radical new system. It did a lot right, and it does more right now that it's matured. But lots of the old are still floating around, getting in the way of new and old players alike.
That said, I don't think they have the company resources to do it. DDI needs manpower, and taking everyone off current projects to do 4.5 for 6-9 months would leave the customer base screaming.