Sources with direct knowledge tell TMZ ... a truck stationed at a facility that Kourtney and co. are using to ship out product for her Lemme brand was stolen from the premises of the warehouse it was at this past weekend, out in the San Fernando Valley here in SoCal.

We're told this happened at the logistics facility Kourtney and her team use for Lemme products -- and this truck that was taken contained upwards of $4 million worth of Lemme Burn ... one of her top-selling products in the Lemme lineup.

In terms of how exactly this happened ... our sources tell us an individual (or individuals) appear to have hacked the facility's system and made their way onto the property with false paperwork and identification ... and we're told they literally drove off the lot scot-free.

Here's the craziest part ... our sources say a ransom note was digitally sent by this group, and it demanded payment to get all these pricey capsules back in Kourtney's possession.

We're told a grand theft report with law enforcement was filed over this ... and cops are now investigating.

A Lemme rep tells us ... "Our team is working closely with law enforcement agencies to investigate the theft. In the meantime, we are implementing contingency plans to minimize any impact to our customers and retail partners."