Incremental salary calculation formulae

Hello,

I am trying to create a formulae that allows automatic calculation of an employee's incentive bonus.

An example of what I am trying to create is as follows:

The employee will receive graduated incentive bonus monthly once their generated revenues reaches a threshold of $4000 in a calendar month as follows:

1. Incentive plan would depend on the following monthly revenue brackets:
   1. 20% for $4,001 to $4,999. (20% of any income that falls within this range).
   2. 25% for $5,000 to $5,999. (25% of any income that falls within this range plus $200 for the range between $4,001 and $4,999).
   3. 30% for $6,000 to $6,999. (30% of any income that falls within this range plus $450 for the range between $4,001 and $5,999).
   4. 35% for $7,000 to $7,999. (35% of any income that falls within this range plus $750 for the range between $4,001 and $6,999).
   5. 40% for $8,000 to $8,999. (40% of any income that falls within this range plus $1,100 for the range between $4,001 and $7,999).
   6. 50% for $9,000 and above (50% of any income that exceeds this amount plus $1,500 for the range between $4,001 and $8,999).

Any suggestions on excel formulae that can be used to determine this calculation utilizing a monthly reference point?

Thank you for your assistance.