We present the ZF2K pilot survey, the first K-band exploration of massive galaxies at 4 < z < 6 using deep (-24.5 AB mag) near-infrared imaging in COSMOS and COFS. The survey employs two medium-band filters that were recently installed on the Flamingos-2 imaging spectrograph at Gemini South. These new filters split the Ks band (centered at 2.2  microns) into two filters - Kb and Kr - centered at 1.9 and 2.3 microns respectively. This modification allows us to better sample the Balmer break of quiescent galaxies, which shifts  into the K-band at z > 4. More importantly, we show that the modification also allows us to reduce by a factor of 7 the fraction of dusty star forming galaxies at low redshift, whose Ks  colors tend to mimic those of z > 4 quiescent galaxies, and improve the photometric redshift scatter by a factor of 4.