I’m happy to explain my reasoning!

Mainly, I liked the fact that marines had T3 in rogue trader, I think it makes them a bit less superhuman and more relatable, which appeals to me. I like that they aren’t better than, say, orks, in every possible way, but actually have a weakness. They are tough and gritty badasses, but they don’t have skin made of steel. Also, my goal isn’t to represent the official 40k fluff/approach in every detail, so no matter how much the fluff may go on about how tough marines are nowadays, in my interpretation of them, they have T3 as standard.
My opponent-friend, who is a die-hard marine fan and has only ever played marines, was a bit uncomfortable at first with this interpretation, but after a few games, he had to admit that the toughness reduction didn’t seem to affect their survivability that much – I still threw almost half my ork army’s firepower at his terminators and only managed to kill one at the end of the game, and the rest of his squads were mostly intact too. So it seems to work out well for us. And since I play chaos marines myself, it’s not just a result of me wanting to take my enemy’s army down a peg!
Ultimately, it boils down to personal preference obviously. If my friend had really wanted them to be T4, I would be happy to tack on a few points and give them their +1T back.