Milestone

you can do it as Tom suggested make it an activity with a 4 month duration however some scheduling specifications will not let you have activities that long in duraiton over 20 days (except for procurement activities). So you can break it down into 4 sperate  activites Phase 1 mobilization 20 days, Phase 2 mobilization 20 days, Phase 3 mobilization 20 days and Phase 4 mobilization 20 days.

or Pre mobilization 20 days, Start mobilization 20 days, continued mobilization 20 days, finish mobilization 20 days.

that will get you your 4 months you can also a a milestone at the beginning and or end of this chain if you would like.