The four TDP parliamentar also met Vice President and Rajya Sabha Chairman Venkaiah Naidu and submitted their resignation from the TDP.
Chowdary, Ramesh and T.G. Venkatesh formally joined the BJP in the presence of its newly-named Working President J.P. Nadda. G. Mohan Rao was not present as he took suddenly ill.
Addressing a press conference, BJP Rajya Sabha member Bhupender Yadav said: "Four TDP Rajya Sabha members held a meeting earlier in the day where they decided to merge with the BJP."
After the four MPs joined the BJP, Nadda said the MPs were thinking for long to follow the dynamic leadership of Prime Minister Narendra Modi and for the development of Andhra Pradesh.
He said all the four MPs are grassroots leaders and they would help in strengthening the BJP in the state.
In their letter to the Rajya Sabha chairman, the TDP MPs wrote that they were "inspired and encouraged by the impeccable leadership of Narendra Modi" and were merging with the BJP.
Earlier in the day, the TDP parliamentary party in Rajya Sabha held a meeting and passed a resolution requesting the Rajya Sabha chairman to immediately merge it with the BJP.
In the resolution submitted to the Vice President, they said they decided to merge TDP parliamentary party in Rajya Sabha with BJP under paragraph 4 of the Tenth Schedule to the Constitution.
Yadav said the four MPs reportedly also met BJP chief Amit Shah during the day with their letter.
With the four TDP MPs merging with the BJP, the tally of the ruling party will go up from 71 to 75 in the Rajya Sabha.
After joining te BJP here, Chowdary said: "The mood of the nation was clear in recent elections."
Chowdary has served as Minister of State in the Modi government till March last year when TDP pulled out of BJP-led NDA over the issue of special category status to Andhra Pradesh.
The development has plunged TDP into a crisis at a time when party President and former Andhra Pradesh Chief Minister N. Chandrababu Naidu is on a foreign trip along with his family members.
Chowdary, a former Central minister and Ramesh are considered close confidantes of Chandrababu Naidu. The two MPs have faced a series of searches by Income Tax, Central Bureau of Investigation and Enforcement Directorate (ED) in recent months.
All the four MPs who defected are businessmen. Chowdary, who owns Sujana group of companies, was named TDP leader in Rajya Sabha a few days ago.
Naidu, who is holidaying in Europe, held a teleconference with the party leaders available at his residence in Amaravati. He advised party leaders and workers not to get worried as the party witnessed and overcame many such crises in the past.
TDP leader and former minister N. Chinnarajappa alleged that the MPs defected to save themselves in various cases.
Interestingly, YSR Congress Party (YSRCP) leaders claimed that the Rajya Sabha members joined the BJP with the consent of Chandrababu Naidu as he wanted to save himself.
The TDP lost power to the YSRCP in Andhra Pradesh in the recent elections. The party could secure only 23 seats in 151-member Assembly. It also won only three out of 25 Lok Sabha seats in the state.
( With inputs from IANS )