Sequence 4 is still in the begining of the game. But killing guards will never be very hard. The more challenging missions are the ones when you cant be detected. But you can make it harder by fighting unarmed and try to take the weapons from everyone in a group that is trying to kill you. ^^ Or steal a poleaxe and knock them of their feet all the time.

Killing in creative ways was more fun than the challenge from the missions imo. Even if you can take down a couple of squads just by rushing in and swinging madly it is a lot more fun to kill them all without anyone noticing you were ever there.