Filed under Rule 424(b)(3), Registration Statement No. 333-275151

Pricing Supplement No. 640 - Dated Monday, July 28, 2025 (To: Prospectus Dated October 24, 2023 and Prospectus Supplement Dated October 27, 2023)

CUSIP Number

Principal Amount

Selling Price

Gross Concession

Net Proceeds

Coupon Type

Coupon Rate

Coupon Frequency

Maturity Date

1st Coupon Date

1st Coupon Amount

Survivor's Option

Product Ranking

63743F3R5

[ ]

100.000%

0.550%

[ ]

Fixed

4.050%

Monthly

08/15/2027

09/15/2025

$4.28

Yes

Senior Unsecured Notes

Redemption Information:Non-Callable

	Offering Date: Monday, July 28, 2025 through Monday, August 4, 2025	National Rural Utilities Cooperative Finance Corp
National Rural Utilities Cooperative Finance Corp	Trade Date: Monday, August 4, 2025 @12:00 PM ET
	Settle Date: Thursday, August 7, 2025	Prospectus dated October 24, 2023 and
	Minimum Denomination/Increments:$1,000.00/$1,000.00	Prospectus Supplement Dated: October 27, 2023
	Initial trades settle flat and clear SDFS: DTC Book Entry only
	DTC Number: 0235 via RBC Dain Rauscher Inc.
	Agents: InspereX LLC, Citigroup Global Markets Inc., Wells Fargo Clearing Services, LLC, RBC Capital Markets, LLC
	Trustee: U.S. Bank Trust Company, National Association
	If the maturity date or an interest payment date for any note is not a business day (as term is defined in prospectus), principal, premium, if any, and interest for that note is paid on the next business day, and no interest will accrue from, and after, the maturity date or interest payment date.
	InterNotes® is a registered trademark of InspereX Holdings LLC. All Rights Reserved.