U100 is 2.5 times more potent than U100 (100/40). Multiply the markings on the syringe by 2.5 when using U100 in U40 syringe. As always, confirm this information with you doctor or CDE.
This may be a dumb question, but why not get a U-100 syringe (or disposable syringes)? The maths of using a U-40 syringe are not that difficult - as @mohe0001 suggests, use 4 u as marked on the syringe, but there’s too much room for error, and in any event you will likely need to adjust your dose at some point. Just get the correct syringes.