The second loadout you described Meckon would weigh more due to the second gun, thus the M4 user has the stamina advantage in that case. Also the nade launcher on the M4 is way faster to access than changing from a SIG to the H&K HK69-A1. Also with all those attachments the M4 would weigh it down giving its full-auto and burst-fire better recoil.
Advantage: M4.
Also I should mention I am a SIG user all the time, so its fair to say that my opinion here is accurate (for the most part, theres speculation here of course)
-EDIT-
The M4 has all the attachments in one place, so the user would not need too much other equipment, so he does not have the large bulk everone is talking about because he will not need to carry many other weapons, in the case of a really skilled user, not anything else. So he has the equipment that others would have if the brought multiple guns - thus he would, in theory, have the stamina advantage if someone wanted to have everything he has.