4 to 4, with no Scalia, is a net win for liberals. Stalling doesn't really hurt the Democrats only for fear of losing the Presidency. Sending forward a liberal nominee, would certainly meet rejection (as you suggest), no matter how well qualified, but it will also serve to turn out the vote, ensuring not only the Presidency but also the Senate; Hillary (or Bernie!) could nominate the same liberal justice with the force of a mandate. Of course there's some risk, but it's not so obvious that Obama shouldn't settle for the single when he could hit something in the gap.