South Belfast MP Alastair McDonnell, South Belfast MLA Conall McDevitt, the current Environment Minister Alex Attwood and mid-Ulster MLA Patsy McGlone have now all declared.
Two candidates are expected to stand for the post of deputy leader - Dolores Kelly and Colum Eastwood.
Mr McDevitt and Mr Attwood confirmed shortly before the deadline on Friday.
Mr McGlone has received ten nominations, Mr McDonnell eight, Mr Attwood seven and Mr McDevitt had six.
A successor to Margaret Ritchie, who stepped down last week, will be chosen at the party conference in November.
Mr Attwood said the SDLP would re-establish itself.
"It will not be the short term before we regain our previous strength. But the power of our ideas, values and the skills of our people remain and are essential to move our politics well beyond the progress of recent years," he said.
Mr McDevitt said: "There is an undoubted mood for real change throughout the party."
He added how he was looking forward to "outlining my vision for a strong, dynamic and progressive party which can reconnect with the many who are currently disillusioned with politics and one which speaks to the hopes, ambitions and dreams of everyone in this region and our island."
Mr McDonnell claimed to be the only candidate who could stop the party's decline and make it electable again.
Mr McDonnell told supporters at a meeting on Friday morning, that he has no intention of presiding over its further decline and eventually disappearance.
He said the party needed to be taken apart and put back together in a form fit for purpose in the 21st century.
Mr McGlone said all candidates agreed on "the need for change within the party".
"It is essential that the SDLP reasserts its objective of uniting the people of Ireland, a unity based on respect for our differences, and my first task if elected will be to outline a pathway to unity for all those living in North," he said.
Dolores Kelly obtained 15 nominations for deputy leader, whereas Mr Eastwood received five.
Four candidates have now put their names forward to stand in the SDLP leadership contest.