Excited states in Si38,40,42 nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of S40 and S44 provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742(8) keV in Si42 confirms the 2 + state reported in an earlier study. Among the γ lines observed in coincidence with the 2 +→0 + transition, the most probable candidate for the transition from the yrast 4 + state was identified, leading to a 41+ energy of 2173(14) keV. The energy ratio of 2.93(5) between the 21+ and 41+ states indicates well-developed deformation in Si42 at N=28 and Z=14. Also for Si38,40 energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for Si42, the results show a rapid deformation development of Si isotopes from N=24 to N=28. © 2012 American Physical Society.