Four staves to one for unison voice

Score is set for SSAA plus piano. Currently the melody line is written on A1 staff. I would like the first 20 measures to be sung in unison. How do I "delete" 3 of the 4 vocal staves for those measures? I've tried following the manual instructions for Implode , but am not achieving the result I need. I basically want one vocal part plus piano for 20 measures, then 4 parts plus piano....thanks for any suggestions!