I need to divest 4 aircraft (Blk 20) from the contract requirements. Can I send a PCOL telling contractor to stop work on 4 aircraft and be done? Do I have to do a contract Mod to remove the requirement for the 4 aircraft or is a stop work sufficient?
One approach to affecting a de-scope via deductive modification is to negotiate a bilateral modification that decreases value of the contract (by impacted option period) relative to the priced cost per aircraft bureau number (that which leads to the "one each" quantity pricing reflected in CLIN/SLIN structure). If appropriate, revise the PWS, CDRL(s) and relevant portions of basic contract terms & conditions (Uniform Contract Format sections) corresponding to changes made in CLIN/SLIN structure.
Your question mentioned a stop-work. Logically, there is no work to stop at current because the option period starts in Jan 2022. Keep in mind that when a Contracting Officer issues a stop-work order (for example using 52.242-15 Stop-Work Order), it's likely the contractor will pursue (partial termination for convenience) "reasonable costs resulting from the stop-work order in arriving at the termination settlement."
It is recommended you discuss this matter with Contracting Officers in your activity to consider how similar situations were resolved and to ensure compliance with internal approvals for this type of contract action.