[AndersHoveland]

I really do not know, but I do not think that elemental phosphorous readily reacts nitrogen directly. There seems to be very little information about this available.

"phosphorous nitride, P3N5
White, odorless, tasteless; decomposes into its elements in vacuum at high temperature. Insoluble in all solvents. Heating with water in a sealed tube to 180 °C decomposes P3N5, forming H3PO4 and NH3. Oxygen affects it (ignition) only at temperatures above 600 °C."
Handbook of Preparative Inorganic Chemistry 2nd ed


Another source described "another form of phosphorous nitride sublimes" towards 1000 °C. The heat of formation of P3N5 was experimentally calculated at "70.4 cals" (-43.9 kcal/mol ?).


"Above 800 °C, P3N5 decomposes, evolving nitrogen: P3N5 --> 3 PN + N2
Phosphorous(III) nitride, PΞN, ... is metastable with respect to decomposition into P2 and N2, PN --> 1/2 P2 + 1/2 N2; Δ= -98 kJ ), but at low temperatures it polymerises into a coloroless phosphorous(III) nitride. In addition to the above compounds, there are intractable yellow to brown nitrides with high melting points and stoiciometries that lie somewhere between PN and P3N5.
"Inorganic Chemistry", Egon Wiberg, A. F. Holleman, Nils Wiberg, p737


None of the sources I have found make any mention to direct reaction between P4 and N2.


"Thus phosphorus nitride is formed comparatively readily only when nitrogen interacts with atomic phosphorus..."
The literature data on the methods of preparation, structures, and properties of phosphorus nitrides PN, poly(phosphorus nitrides) (PN), (P3N5), ... are surveyed and their practical applications, formed as a result of the direct binding of molecular nitrogen... are examined.
"Phosphorus Nitrides", Evgenii V Borisov and E E Nifant'ev
 

I suppose the ideal experiment would bring vaporised P4 at a high temperature in contact with nitrogen gas, then see if any PN polymer formed as a powder. Simply trying to heat solid phosphorous with nitrogen would not work, because the nitride would potentially form a protective coating, with a high decomposition temperature, that would inhibit further reaction.

[AndersHoveland]

I would think that P4 and N2 could be reacted together at some high temperature. Unlike the reaction between H2 and N2 to form ammonia, the polymer product PN may have a high disassociation/vaporisation temperature, which would presumably shift the equilibrium forward. In other words, high pressure may not be required. The formation of NH3 only requires high pressures because at such high temperatures (beginning above around 455 °C ) NH3 has a tendancy to decompose back to its elements, whereas P3N5 apparently does not decompose until around 800 °C.
 
However, if direct combination of the elements were possible, one would wonder why all the methods of preparation for P3N5 use other more complicated routes. While the formation of P3N5 from the elements is energetically favorable, it is only slightly so; not much energy is released. Because of this, it is quite possible attempts at such a reaction would be overwhelmed by other unfavorable factors, such as Le Chatelier's principle. The formation of the polymer phosphorous(III) nitride, PN(n) may be more energetically favorable, but this lower nitride also seems to have a lower boiling point (disassociation into diatomic PN), which would not help favor its formation.