First, you don't have only 4-7 years to save... Assuming 4 year degrees, you really have 7-11 years to save -- The colleges accept money on a term by term basis... you don't pay for the final 4th year until you enter that year...
So, in 11 years, you want to save (and spend) $160k (2 students x 4 years at $20k per year parental contributions each for live away, in state costs)...
OR ... do you want to save $90k, assuming that they can live at home on your dime, you pay tuition, and they fund their own residence fees?
That means that you need to save $$14.5k per year (High) to $8.2k per year (Low).
Can you free up $10k per year from your budget?
If not, you may need to look into community colleges, FAFSA loan potential, and other loans, or delay entry into college by a couple of years (and they can help work and save for college too). They can work summers to offset later years' costs, too. Most will be able to save $4000 minimum over a summer, so $12k could pay for the final year of in-state tuition, etc .
Lots of options, even though you are only 4 years away. The key is to have at least the first year fully saved before they start.