Insert the second screw
Remove the K-wire.
As a rule, a screw which is 5 mm longer than the measured size must be chosen for the second screw as the more medial portion of even a large Volkmann's fragment is often not deep enough to accommodate the whole of the threaded portion of the cancellous screw.
The screw tip may therefore protrude a little posteriorly.
If the thread of the 4.0 mm partially threaded screw would still come to lie on both sides of the fracture, or to prevent protrusion of the tip of the screw into the posterior soft tissues, insert a 3.5 mm cortex screw as a lag screw.