A search for the owners is underway after the bulldogs were pulled from Southmere Lake in Thamesmead.
Director Vicki Hudson thought the bags suspicious and called the police who sent a marine unit to recover the animals.
The dogs are believed to be a one-year-old bulldog puppy and three adult extra large bulldogs, aged around two years.
Properly certified and healthy bulldog pups can sell for up to £5,000.
Another puppy was also found on Sunday at the other side of the lake but it is not believed to be connected.
Ms Hudson said the reason for the dogs being dumped will likely only emerge after speaking with the owners following the find at 9pm last Saturday.
She said: “Unless we can locate the person who put them in there and find out what the story was, or someone comes forward and says my neighbour had dogs that disappeared.
“We are still trying to do background traces for owners. The chips were not flagged as stolen. There is nothing obvious.
“How or why they ended up in the water I don’t know. I’m banging my head against the wall.”
She added: “They are expensive dogs so if you are breeding, the average pup would be £4,000-£5,000, depending on the blood line, certificates and the dog was in good condition.
“If you were a breeder, why would you get rid of your stock? And if you are an owner, there are places you can take your dog to.”
“We have got to wait for the post-mortem but I think they were dead before they went into the water.
“There were no obvious signs that they had struggled in the bags.”
The 51-year-old said another dog had been found on Thursday buried in a shallow grave and that they found the puppy on the other side of the lake after another call from the public.
She said: “We don’t think that is connected because it has been presented in a different way.
“But people are disposing of pets in that lake and it’s something we need to keep an eye on.
“We have been going for around five years now. Every few months we see a body pop up.
“A member of the public will call us and say they have seen a body covered or partially covered in a bag.
“In this area it has gotten more regular in the past six months. I’m not sure whether that is deprivation or someone breeding dogs, I don’t know.”
Ms Hudson added: “If a person was to find themselves in financial difficulty, then please reach out to us or the RSPCA or charity kennels, but don’t do this. It is not the right way.”
RSPCA chief inspector Mark Miles added: “This is clearly a very disturbing discovery and we’re very keen to find out more as soon as possible.
“To find four dead dogs of similar breeding known as ‘XL American Bullies,’ in the same location suggests they were dumped by the same owner or breeder - someone who is possibly known in their community for owning this type of large dog, with cropped ears.
“So far we believe that one of the dogs, a brown and white female American bull breed, was approximately seven months old, and from details on her microchip we believe her name was Luna. We’re still finding out more about the remaining three dogs.
“We continue to work with the Metropolitan Police to investigate this horrific incident, and we’re urging anyone with information to contact the RSPCA’s inspectorate appeal line on 0300 123 8018. I’d like to reassure people they can contact us in complete confidence.”
The Met Police has said it is waiting for the results of the post-mortem before making any further decisions or comments.
Any potential witnesses who were near Bazalgette Way by Southmere Lake on Saturday evening are urged to speak with either the Met Police or Missing Paws UK.