The Boston Celtics have agreed to a four-year, $113-million max deal with free-agent center Al Horford, sources told The Vertical's Adrian Wojnarowski.

Horford wasted little time Saturday in announcing the news to the world.

The fourth year of the contract is a player option, ESPN's Ramona Shelburne reports.

The Atlanta Hawks - with whom Horford has spent his entire nine-year career to date - reportedly made a spirited last-ditch push to retain him after the two sides failed to come to an initial agreement. The Hawks owned Horford's Bird rights, allowing them to offer him a fifth year, but they were reluctant to offer him the full max he was seeking.

In the end, Horford took less guaranteed money from the Celtics, but at a higher annual value. The Hawks' final offer came in at five years, $136 million, sources told ESPN's Brian Windhorst.

Wojnarowski reports the two sides were just $6 million apart by the end of their negotiations.

The Hawks would've had to make a trade (or several) to clear cap space for a Horford contract after signing center Dwight Howard and swingman Kent Bazemore to sizeable free-agent deals. They were apparently prepared to do so, and engaged in trade talks involving power forward Paul Millsap before Horford made his decision, but will now hold onto Millsap.

The Celtics, meanwhile, get the difference-making star they've long been seeking, but without having to dip into their impressive bounty of assets. Horford fills a clear position of need in the Boston frontcourt, giving them a dynamic, mobile, two-way big with the ability to pass, post up, and space the floor.