  We report high precision photometry of three small and one larger Kuiper Belt
Objects (KBOs) obtained with the Advanced Camera for Surveys onboard the Hubble
Space Telescope (ACS/HST). The three small bodies are the smallest KBOs for
which lightcurve measurements are available. 2003 BF91 has a diameter of 20
kilometers (assuming 10% albedo) and a 1.09 magnitude, 9.1-hour lightcurve that
is feasibly explained by the rotation of an elongated, coherent body that is
supported by material strength and best imagined as an icy outer Solar System
analog to asteroid (243) Ida. Two other small KBOs, 2003 BG91 and 2003 BH91
(diameters 31 and 18 km, with albedo 10%), exhibit an unremarkable lightcurve
and no detectable photometric variation, respectively. For the larger KBO 2000
FV53 (116 km diameter, assuming 10% albedo) we strongly detect a non-sinusoidal
periodic (7.5 hours) brightness variation with a very small amplitude (0.07
mag). This KBO may be nearly spherical, a result that might not be unusual in
the Kuiper Belt but would be remarkable among outer Solar System satellites of
similar size. We carry out a study of possible physical states and bulk
densities under the assumptions of both fluid equilibrium and finite, non-zero
internal friction. The densities for the these KBOs are likely to be in the
range 1--2 g/cm3, and a plausible solution for 2000 FV53 is a rubble pile of
this density that is held slightly out of the minimum-energy shape by internal
friction among constituent blocks that are relatively small. Our interpretation
of 2000 FV53 as a pulverized but essentially primordial object and 2003 BF91 as
a collisional fragment is consistent with models of collisional timescales in
the outer Solar System. We compile all published KBO lightcurve data and
compare our results to the larger population. [abridged]
