Check out this article on lags/predecessors for help. Month isn't indicated in here but it could maybe work with 'm' instead of 'w' for weeks, or you could use a predecessor for 4 weeks. For example, if you want a task due the first Monday in 4 weeks after task 5, I think it would be 5fs+42w (42 is 4=4wks and 2=the # assigned to Monday in lags).
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