  We take deep images of four ultra faint dwarf (UFD) galaxies, Canes Venatici
I (CVn I), Bootes I (Boo I), Canes Venatici II (CVn II), and Leo IV using the
Suprime-Cam on the Subaru Telescope. The colour-magitude diagrams (CMDs) extend
below the main sequence turn-offs (MSTOs) and yield measurements of the ages of
stellar populations. The stellar populations of faint three galaxies, Boo I,
CVn II and Leo IV dwarf spheroidal galaxies (dSphs) are estimated as old as a
Galactic globular cluster M92. We confirm that Boo I dSph has no intrinsic
colour spread in the MSTO, and no spatial difference in the CMD morphology,
which indicates that Boo I dSph is composed of an old single stellar
population. One of the brightest UFDs, CVn I dSph, shows a relatively younger
age (-12.6 Gyr) with respect to Boo I, CVn II, and Leo IV dSphs, and the
distribution of red horizontal branch (HB) stars is more concentrated toward
the centre than that of blue HB stars, suggesting that the galaxy contains
complex stellar populations. Boo I and CVn I dSphs show the elongated and
distorted shapes. CVn II dSph has the smallest tidal radius as a Milky Way
satellite and has distorted shape, while Leo IV dSph shows less concentrated
spherical shape. The simple stellar population of faint UFDs indicates that the
gases in their progenitors were removed more effectively than those of brighter
dSphs at an occurrence of their initial star formation. This is reasonable if
the progenitors of UFDs belong to the less massive halos than those of brighter
dSphs.
