We report the first results of a spectroscopic search for Ly alpha envelopes around three z similar to 4.5 radio-quiet quasars. Our observational strategy adopts the FORS2 spectrograph mounted to the UT1 of the Very Large Telescope (VLT) in the multi-slit mode. This allows us to observe simultaneously the quasars and several PSF stars. The spectra of the latter are used to remove the point-like quasar from the data, and to unveil the faint underlying Lya envelopes associated with the quasars to unprecedented depth. We clearly detect an envelope around two of the three quasars. The sizes of these envelopes are 10 '' and 13 '' (i. e. 67 kpc and 87 kpc). This is 5 to 10 times larger than predicted by the models of Haiman & Rees (2001, ApJ, 556, 87) and up to 100 times fainter. Our observations are in more robust agreement with models involving a clumpy envelope such as Alam & Miralda-Escuda (2002, ApJ, 568, 576) or Chelouche et al. (2007, ApJ, submitted). We find that the brighter quasars also have the brighter envelopes but that the extent of the envelopes does not depend on the quasar luminosity. Although our results are based on only two objects with a detected Lya envelope, the quality of the spatial deblending of the spectra lends considerable for hope to estimating the luminosity function and surface brightness profiles of high redshift Ly alpha envelopes down to F similar to 2-3 x 10(-21) erg s(-1) cm(-2) angstrom(-1). We conclude that the most efficient strategy for studying high redshift Lya quasar envelopes is to acquire both narrow-band images and deep slit-spectra.