As mentioned by others, use Permit functionality for the cost approval.
Create 4 different permits & give the authorization to these permits according to your requirement using I_SOGEN. Once the planned cost goes beyond certain limit or as per your requirement, Permits can be assigned automatically.
IWO10007 Maint.order: Customer enhancement - permits in the order
IWO10009 PM Order: Customer Check for 'Save' Event
You can use above User Exits to trigger the mail.
Activate the workflow & for dynamic selection of receivers of mail (cost approvers) you can write the coding to read the respective person.
IMG - PM & CS - Maintenance & Service Processing - Maintenance & Service Orders - Functions & Settings - Set Workflow for Orders