  Hadronic events from the data collected with the DELPHI detector at LEP
within the energy range from 89 GeV to 209 GeV are selected, their jet rates
are determined and compared to predictions of four different event generators.
One of them is the recently developed APACIC++ generator which performs a
massive matrix element calculation matched to a parton shower followed by
string fragmentation. The four-jet rate is used to measure alpha_s in the
next-to-leading-order approximation yielding alpha_s(M_Z^2) = 0.1175 +/-
0.0030. The running of alpha_s determined by using four-jet events has been
tested. The logarithmic energy slope is measured to be d\alpha_s^{-1} / d\log
E_{cm} = 1.14 +/- 0.36. Since the analysis is based on four-jet final states it
represents an alternative approach to previous DELPHI alpha_s measurements
using event shape distributions.
