Of the three officers, the most well-known is Maj. Hamza al-Mustapha, the former security chief under late Nigerian dictator Sani Abacha, the BBC reported.
He has been in custody on a different murder charge, but the prosecution alleges he held prison meetings to plot the coup against Obasanjo.
All four have pleaded not guilty to the treason charges. Until now, the only government reference about the April coup plot was that there was a "serious breach" of national security.
The charges accuse al-Mustapha of financing efforts to buy "a Stinger surface-to-air missile to be used in shooting down the president's helicopter with the president on board." Nigeria returned to civilian rule in 1999 -- following years of military dictatorship.