  In this fourth part of the series presenting the Optical Gravitational
Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact
objects (MACHOs) we describe results of the OGLE-III monitoring of the Small
Magellanic Cloud (SMC). Three sound candidates for microlensing events were
found and yielded the optical depth tau_SMC-OIII=1.30+-1.01 10^{-7}, consistent
with the expected contribution from Galactic disk and SMC self-lensing. We
report that event OGLE-SMC-03 is the most likely a thick disk lens candidate,
the first of such type found towards the SMC. In this paper we also combined
all OGLE Large and Small Magellanic Cloud microlensing results in order to
refine the conclusions on MACHOs. All but one of OGLE events are most likely
caused by the lensing by known populations of stars, therefore we concluded
that there is no need for introducing any special dark matter compact objects
in order to explain the observed events rates. Potential black hole event
indicates that similar lenses can contribute only about 2 per cent to the total
mass of the halo, which is still in agreement with the expected number of such
objects.
