  The evolution of massive stars is only partly understood. Observational
constraints can be obtained from the study of massive stars located in young
massive clusters. The ESO Public Survey VISTA Variables in the Via Lactea (VVV)
discovered several new clusters hosting massive stars. We present an analysis
of massive stars in four of these new clusters. Our aim is to provide
constraints on stellar evolution and to better understand the relation between
different types of massive stars. We use the radiative transfer code CMFGEN to
analyse K-band spectra of twelve stars with spectral types ranging from O and B
to WN and WC. We derive the stellar parameters of all targets as well as
surface abundances for a subset of them. In the Hertzsprung-Russell diagram,
the Wolf-Rayet stars are more luminous or hotter than the O stars. From the
log(C/N) - log(C/He) diagram, we show quantitatively that WN stars are more
chemically evolved than O stars, WC stars being more evolved than WN stars.
Mass loss rates among Wolf-Rayet stars are a factor of 10 larger than for O
stars, in agreement with previous findings.
