I think <PERSON>'s ‘green martians’ were probably one of the better designs for this. E.g. Tars Tarkus.
Here are some images from the 2012 cinematic version:
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Here is Tars on the cover of ‘Gods Of Mars’:
I did not remark the source for this one.
Here's a version which doesn't resemble the original depictions so much as do those from the 2012 movie, but which does show the characteristic structure as I wish to proffer — albeit exaggerated and rather bulky:
Here is one more for sake of variety:
Source: http://the-first-magelord.deviantart.com/art/Tars-Tarkas-WIP-255092129
My idea has always been this: If you wish to add a pair of extra arms, then simply extend the height of the thorax by addition of clavical units.
To reduce the total height, you redistribute and reshape the internal organs to accomodate the new bones — all as compared to the human structure.
Of course — and similarly to what was stated in this answer, — you will need to provide additional scapulae and sternum on which to anchor the muscles for the arms.
Flexibility and strength of any limbs depends, of course, on the mechanical advantages of their leverage in relation to the centers of mass.
Well, anyway, it seems that the Green Martian design overcomes the problems with crowding extra arms on a humanoid frame by simply adding height: the original descriptions put them at 10 or so feet (3 m) tall.
The curvature of their vertebra would need to be somewhat different from ours — and not simply stretched vertically, nor like two torso stacked one atop the other. 321
The added load atop the bottom vertebra would probably require that they be broader so as to distribute the forces over a larger area, for one thing. Too much curvature at the bottom would be difficult when maintaining an upright posture, so you'd probably sacrifice some flexibility there for better stability and support.
Unless you want to go for some vertebrate polypede thing, such an approach does limit you to one additional pair for concern of height.
It should be noted that the original description of these Green Martians also allowed them to use their intermediary limbs as legs, if they so need. Thus, their bones were probably not quite like additional shoulders so much as a hybrid between shoulders and hips.
Furthermore, note that not all depictions of <PERSON>'s Green Martians have been portrayed so novelly. This one looks like a humanoid, and the lithographer obviously was skilled, but the extra arms are carelessly applied: 249