Sorry can't PM back as I haven't made 30 posts yet. Yes it was me that spoke to GMK, small world.
Perhaps other people can chip in as well as I'm new to Benelli's and PSG. My understanding with the Benelli M2's is that in order to extend the mag without gunsmithing you need the shortmag version. According to Wikipedia (a well known shotgun authority

http://en.wikipedia.org/wiki/Benelli_M2
The tactical or longmag is 5+1 and crimped so I think you need a hacksaw to extend.
I haven't come across 4+1 and don't know why these would be chosen over a 3+1. If they should be please let me know asap as I have placed an order for 3+1 field. Nordic (when they are available) do extensions from +1 to +8 so you could extend to 10 with either.
http://www.blackguns.co.uk/nordic_shotgun.html
Alex.