I'm not sure what you mean by magnetisation of cables, but I believe you may be referring to one or both of two effects.
First, a single conductor cable will induce current in surrounding ferrous metal. If the cable is carrying enough current, this can cause substantial heating of the surrounding metal. This means the installer should use non-ferrous cable straps and non-ferrous entry plates at equipment.
Second, asymmetrical arrangement of parallel conductors causes differences in inductive reactance for each cable, with a resulting unequal division of load current. You don't say how you are running your cables, but if you were running them in two spaced layers in cable tray, or in a (non-ferrous) duct bank, a possible arrangement could be A1-B1-C1-C2-B2-A2 in the top layer and A3-B3-C3-C4-B4-A4 in the bottom layer. Alternatively you could run four (non-ferrous) ducts, each with an A-B-C arrangement. I'm from Canada and our Electrical Code illustrates various arrangements for parallel cables, so perhaps checking the Electrical Code for your area will be of help.
I'm not an expert on running the neutral as most of my work is on industrial systems with resistance-grounded system neutral and no single-phase loads, but I assume if it carries return currents there is no harm in treating it as another phase in grouping cables. On the other hand, since the return currents are zero sequence, you may be able to group all the neutral conductors together and run them separately. Anyone out there know more about this?