Ernesto Sierra, Jorge Caso, Sergio Lorenzo and Andres Dennis were identified as the victims, according to the group.
The Americans, all from the Miami, Florida area, were among a group of 14 friends visiting the Central American nation for a bachelor party.
The local guide was identified as Kevin Thompson Reid.
"We've all been friends for many years and we knew we wanted to do something special for our friend before his wedding".More news: More than 2 million pounds of taquitos being recalled
"We have had some trouble confirming the total number of tourists and their nationalities since we have been unable to get a hold of the representatives of the tour company", local outlet the Costa Rica Star quoted him as saying.
Thirteen people survived by holding on to the overturned rafts but five were pulled farther down the river.
Costa Rica has seen heavier than usual rainfall recently, causing the waters in the Naranjo River to rise.
Castro said the river's "immense current" kept flipping the rafts as he and others tried to pull themselves out of the water.More news: Amy Schumer is pregnant and wants you to vote on November 6
"Throughout the unsafe ride down the river, all of us struggled to stay above water, swallowing lots of it on the way down as our bodies ricocheted against the rocks in the water while struggling to survive".
"They all had so much life left to live as sons, brothers, fathers cousins and friends before this devastating tragedy occurred", Castro said in the GoFundMe posting. "Four of our friends drowned in those waters". Comas said that one of the victims, Lorenzo, was the groom's brother.
Four American tourists and their tour guide have been killed in a weekend rafting accident in Costa Rica.
"I would like to thank all the emergency and security teams that worked diligently, and I have instructed authorities to help the victim's families".More news: Suspect Found, Killed 3 Days After He Allegedly Killed Cop