Well…before you do this…you need to determine how much you can actually afford to pay for college for each if four years.
Then you need to run those net price calculators. View them as an estimate of your net costs.
If you are self employed, own a business, own real estate other than your primary residence, or parents are divorced…the NPC is less accurate. Also, when will your kid start college because the NPCs are currently set for students starting in fall 2019.
In terms of subsequent college years…you apply for aid annually using prior prior year tax year information. So for 2019-2020 you would be using 2017 info. If your financial picture changes, your need based aid will probably change too…at least at schools that guarantee to meet full need for all. At schools that don’t guarantee to meet full need, you should have no expectation of increased aid if your income goes down in subsequent years…you might get more need based aid at schools that don’t meet full need…but you might not.
I would suggest adding 3% or so to the annual cost of attendance at schools for subsequent years.
And biggest caveat. If your kiddo is applying to colleges that guarantee to meet full need, MANY (especially the most generous) have single digit acceptance rates…so admission is the first hurdle.
What colleges are your student considering?
ETA…if you think you can pay $4000 a month…start putting that amount into a dedicated savings now…and see if you feel this is sustainable for four years.