Region IV-A, a region of great natural beauty that is also known as Calabarzon and more formally called the Southern Tagalog Mainland. It consists of five provinces wherein the word Calabarzon is comprised of one or more letters of each province's name. CAvite, LAguna, BAtangas, Rizal and QueZON. The region is in southwestern Luzon, just south and east of Metro Manila and is the second most densely populated region in the country.

The region has travelled a long way from its first creation sometime in 1973 when it was originally under the territorial jurisdiction of Region 3A which consisted of the Provinces of Cavite, Laguna, Batangas, Marinduque, Oriental Mindoro and Quezon under the leadership of our first Regional Director Engr. Federico L. Gendrano (1973-1976 & 1984-1986). In 1977, regrouping of provinces under the Regionalization Scheme was implemented wherein Region 3A has become Region 4 and five Provinces were added, namely; Occidental Mindoro, Palawan, Aurora, Rizal and Romblon headed by Regional Director Engr. Romeo S. Roque (1977-1978) followed successively by Engr. Florentino A. Camarador (1978-1983), Engr. Nicasio T. San Miguel (1986-1990), Engr. Crisanto A. Gimpaya (1990-1991), Engr. Rolando T. Bonrostro (1991-1997), Engr. Baltazar H. Usis (1997-2005), Engr. Alberto G. Dela Cruz (2005-2006), Engr. Efren S. Roqueza (2006-2011) up to the present direction of Engr. Romeo R. Aņonuevo. Many years have passed and the region has worked hard to give good service to its farmer-beneficiaries for a worthy efficiency return on ISF collection and CIS amortization collection. 

On April 27, 2011, by virtue of EO 103, separation of Region IV into IV-A and IV-B was effected. During this time, the NIA implementation of Rationalization Plan is on-going (2008-2012) and Region IV was rationalized to a six Irrigation Management Offices (IMOs). After the splitting of Region IV, the six IMOs were divided into two. Region IV-A has a jurisdiction on Laguna-Rizal IMO, Cavite-Batangas IMO and Quezon IMO. 

At present, Region 4A has a total land area of 1,641,303 hectares, that has a total arable area of 912,411 hectares. Potential irrigable area is 97,710 hectares while the current percentage of Developed Area is 59.77% or a total of 58,401 hectares. Irrigation systems in the region are divided into 39 National Irrigation Systems (NIS) and 339 NIA-assisted Communal Irrigation Systems (CIS). There are 83 organized NIS Irrigators Associations (IAs) and 240 CIS IAs who have been a consistent partner in irrigation development.
CY 2012 had been a blessed year for NIA, as for the first time it attained its corporate financial viability together with the whole Regional Offices nationwide. Indeed, Region 4A is pleased that even a considered "small" region has contributed to the agency's financial viability.