#4 is the shear bolt. #14 is the stud that screws into and through the universal joint yoke, and then into the groove in the input shaft to the gearbox; when the shear bolt shears and the yoke spins around the shaft, this stud keeps the yoke from slipping off the shaft because the stud extends into the groove and just spins around in it.
Shear bolt breakage is a common occurrence with PHD's; I actually built a little bracket that bolts to the gearbox on my Bush Hog PHD to hold spare shear bolts.
If you can't find a factory replacement, just get a grade 3 hex bolt the same size as the sheared bolt; grade 3 is the weakest grade and it is unlikely the factory bolt would have been made of anything with less shear strength. In my experience, the shear bolts on PHD's are to protect the drive shaft; they are fairly long to reach from the tractor PTO to the gearbox when the PHD is raised completely, and to save money the manufacturers use a light grade of drive shaft.
A few manufacturers use a shear bolt with a reduced size on the thread end; I am not sure whether that is to drive you back to the dealer for replacements or to discourage you from using hex bolts from the hardware store because you might get a grade 5 or 8 bolt that is much stronger than the grade 3 and might not shear before the driveshaft were damaged. If that is the case, just drill out the smaller hole in the yoke to the same size as the larger hole in the other side of the yoke.