  We present OASIS observations obtained at the Canada-France-Hawaii Telescope
for the SB(rs)c galaxy NGC 4900. About 800 spectra in the wavelength range
4700-5500 AA and 6270- 7000 AA have been collected with a spatial resolution of
~50 pc. This galaxy is part of a sample to study the stellar populations and
their history in the central region of galaxies. In this paper, we present our
iterative technique developed to describe consistently the different stellar
com- ponents seen through emission and absorption lines. In NGC 4900 we find
many young bursts of star formation distributed along the galaxy large scale
bar on each side of the nucleus. They represent nearly 40 per cent of the
actual stellar mass in the field of view. The age for these bursts ranges from
5.5 to 8 Myr with a metallicity near and above 2 Zsun . The extinction map
gives E(B-V) values from 0.19+/-0.01 near the youngest bursts to 0.62+/-0.06 in
a dusty internal bar perpendicular to the large scale bar. The Mg 2 and Fe I
absorption lines indicate the superposition of a background stellar population
with an age between 100 Myr to 3 Gyr and a sub-solar metallicity on average. We
propose that all these episodes of star formation are the consequence of a
secular evolution. In this scenario, the galactic large scale bar plays an
important role with respect to the recent bursts and the dusty nuclear bar
observed. The iterative technique allows us to improve the determination of the
stellar population parameters, mainly an older age is obtained for the old
component and more reliable stellar population masses are found. A
composite/transition type activity in the galaxy nucleus is also revealed with
this technique.
