Might sound a bit strange but I would look at how spiders legs are attached (if that makes sense). As they have multiple limbs that are attached in close proximity.
You might have to extend the body slightly to be honest as it would llok a bit strange if you didnt, also it probably wouldbe more anatomicaly correct (for a 4 armed mutant!!) just some things to keep in mind.
Cheers
Steve
"No pressure, no diamonds" Thomas Carlyle