Would-be rescuers attempting to pull out people who had fallen into the sea had to then be saved by passersby.
A man fell from the slipway at Dundas Street in Redcar at about 16:00 BST on Good Friday and was battered by large waves close to the sea wall.
A man he was with attempted to rescue him but also got into difficulty. A man and a woman who attempted to rescue the pair then also ended up in the sea.
Members of the public were able to get all four to safety by forming a chain.
Redcar RNLI said it was only through "good fortune" they were all saved and urged against copycat behaviour.
Spokesman Dave Cocks said: "This was a sequence of events which could so easily have led to multiple deaths.
"The sea at Redcar is very rough as a result of the combined effects of strong winds and a particularly high tide, and the situation ended up with four adults in trouble in the sea, all needing rescue.
"It is only through good fortune that all four were rescued alive.
"If you see someone in trouble, alert the lifeguards... do not enter the water yourself."
All four were taken to James Cook University Hospital in Middlesbrough for treatment.