In reply to Reach>Talent:
This probably echoes what your 3rd spark said:
3PN+E can be wired with 4c armoured cable using the armour for earth. 5c is typically (but not exclusively) used where the earth loop impedance is too high using the armour alone. Even then a supplementary or separate cpc (earth) is commonly used because 5c cable can be relatively expensive.
Some three phase kit doesn't require a neutral. The equipment should identify if this is the case. If so 4c or 3c + (armour and/or separate cpc) can be used.
Ground return systems for the neutral are not recognised by BS7671 (nor I believe the German DIN equivalent). Some railway traction systems use them but they are covered by different standards. Jimtitt's 4 wire vs 5 wire observation most likely reflects the utility supply earthing system, e.g. TT vs TN-C-S vs TN-S, which is not directly applicable to the OP. It does affect the earth loop impedance requirements though so influences at what point the armour alone becomes insufficient as the cpc.