  We present observations of a confined M4.0 flare from NOAA 11302 on 2011
September 26. Observations at high temporal, spatial, and spectral resolution
from Solar Dynamics Observatory, Reuven Ramaty High Energy Solar Spectroscopic
Imager, and Nobeyama Radioheliograph enabled us to explore the possible
triggering and energy release processes of this flare despite its very
impulsive behavior and compact morphology. The flare light curves exhibit an
abrupt rise of non-thermal emission with co-temporal hard X-ray (HXR) and
microwave (MW) bursts that peaked instantly without any precursor emission.
This stage was associated with HXR emission up to 200 keV that followed a power
law with photon spectral index ($\delta$) $\sim$3. Another non-thermal peak,
observed 32 s later, was more pronounced in the MW flux than the HXR profiles.
Dual peaked structure in the MW and HXR light curves suggest a two-step
magnetic reconnection process. Extreme ultraviolet (EUV) images exhibit a
sequential evolution of the inner and outer core regions of magnetic loop
system while the overlying loop configuration remained unaltered. Combined
observations in HXR, (E)UV, and H$\alpha$ provide support for flare models
involving interaction of coronal loops. The magnetograms obtained from
Helioseismic and Magnetic Imager (HMI) reveal the emergence of magnetic flux
which started $\sim$5 hr before the flare. However, the more crucial changes in
the photospheric magnetic flux occurred about 1 minute prior to the flare onset
with opposite polarity magnetic transients appearing at the early flare
location within the inner core region. The spectral, temporal, and spatial
properties of magnetic transients suggest that the sudden changes in the
small-scale magnetic field have likely triggered the flare by destabilizing the
highly sheared pre-flare magnetic configuration.
