Rockets will offer max that they can (something around 4 years and 80 right?) So the C's would have to match that at least, and I wouldn't be shocked if he'd threaten to walk to play with Harden if he didn't get the fifth year.
So you match, but remember Celtics can offer higher raises than them per year. I'm not saying that a fifth year isn't a possibility, but to think that a deal for a 4 year contract is impossible as is being insinuated is simply an inaccurate assessment of the situation, particularly by the apparent aversion from a lot of teams of committing to a max deal for Smith.
Houston indeed is the most worrying, and actually Atlanta themselves could be worrying if they get Rondo... not sure if they'd want Smith back though.
The most likely scenario in my opinion is that Ainge makes a trade if a negotiation for an extension takes place alongside it.