The preliminary expense budget shall contain proposed expenditures and a forecast of revenues for the ensuing fiscal year, including, for each tax revenue source which represents five percent or more of the total forecast of tax revenues, a detailed statement of the methodology and assumptions used to determine the forecast of revenues estimated to be received from such source in sufficient detail to facilitate official and public understanding of the manner in which such forecasts are made, shall indicate proposed units of appropriations for personal service and for other than personal service, shall include a financial plan for the city for the four ensuing fiscal years (of which the first year is the year for which such preliminary expense budget is being prepared), consistent with section two hundred fifty-eight, with the amounts estimated to be available for discretionary increases, as defined in section one hundred two, in such years, shall include the departmental estimates of agency expenditures for the ensuing fiscal year pursuant to section one hundred together with proposed sources of revenue for each unit of appropriation specified therein and shall present a plan to ensure balance between the expense and revenue budgets during the ensuing fiscal year.