Okay, I know marines are nerfed when it comes to the table top anyways. But assuming for a minute that their stats actually represent the way they're portrayed in the fluff...

a) Would it not make more sense for a marine to be T3, Feel No Pain?

I mean, as far as I can tell, a marine with his armor off in the fluff is just as vulnerable to wounds as a regular human, but his enhanced physiology allows him to ignore wounds that would incapacitate a human (like the loss of an arm) but would nevertheless render him useless in combat?

So, marine takes a bolter to the chest, even a las shot, he's as dead as a normal human; or a sword cuts removes his arm as easily as it would a regular human's. Thus: T3. However, because his enhanced physiology (extra vital organs, rapid staunching of blood loss, psychological resistance to pain, etc) allows him to easily ignore wounds that aren't instantly fatal. Thus: Feel No Pain.

b) Could the T4 be rationalized by Astartes-grade power armor's natural resilience (which also explains away why Inquisitors/SoB/etc don't get the boon, as it's not Astartes-grade) that grants the T4? That doesn't make sense because Scouts have T4 too. I suppose the Black Carapace could warrant the Toughness increase, but Scouts don't have it either. Hmm...