The last four suspects, including the head of military intelligence at the port, two General Security personnel, and a major at State Security, were questioned and then arrested yesterday.
AFP named the latest detainees as Brigadier General Antoine Salloum, Major Daoud Fayyad, Major Joseph Al-Naddaf, and Major Charbel Fawwaz.
A further 21 suspects had already been detained as part of the probe, including the Director-General of Beirut Customs Badri Daher and the Director-General of Beirut Port Hassan Qureitem, who were arrested two weeks ago.
Many of the 25 in custody were put under house arrest in the days after the 4 August blast, pending questioning and official detention. It is not clear what the next step in the investigation will be.
A public outpouring of anger after the blast led to popular claims negligence and corruption was to blame for the explosion.
Government and port officials who, documents unearthed after the blast show, were aware tonnes of highly explosive chemicals were stored unsafely only hundreds of metres from the city, have borne the brunt of these claims.
Officially, however, the circumstances surrounding the blast remain unclear.
Lebanese security forces have advanced the theory sparks from a welding iron used to carry out maintenance on Beirut port's warehouse 12 hours before the explosion caused the chemicals to ignite, but the hypothesis has been rejected by many observers.
The explosion, which happened when the 2,750 tonnes of ammonium nitrate caught light, caused widespread material damage as far as 15 miles aware from the site of the blast, injured more than 6,500 and claimed the lives of nearly 200.
One of those who died as a result of last month's explosion was 98-year-old Lady Yvonne Sursock Cochrane, a prominent art and culture advocate in Lebanon, who succumbed to wounds sustained in the blast, on Monday evening.
Lady Cochrane, as she was known after her marriage to Irish nobleman Desmond Cochrane in 1946, was a popular defender of Lebanon's heritage and co-founded the Association for the Protection of the Natural Sites and Ancient Buildings.
She was also heavily involved in the development of Beirut's famous Sursock Museum in the 1960s, after the building was bequeathed by her uncle, Nicolas Ibrahim Sursock, to the city of Beirut after his death in 1952.