I can't see anything in there to even pause over, let alone look in a book.
Roll attack
12
Roll damage
14
The arrow pierces the goblin's armour. It's body pirouettes and pitches forward into the moat.
Check to see if the splash is heard.
The guard at the gate looks over - but not in time to see the corpse hitting the water.
Thief tries to get past the guard without being noticed.
Done
Well, 14 damage won't kill a goblin (other than a minion) in 4e. One option, of course, is to make the sentry a minion. On the other hand, suppose that things go wrong and the PCs find themselves confronting 5 sentries who have assembled in formation, I'm not sure I want to treat that as a combat vs 5 minions - it would be more interesting as a combat vs 5 real goblins.
If I do treat the sentry as a minion, then how does killing it factor into the skill challenge? A success? A +2 to someone else's Stealth check? This is the sort of mechanical advice I think a rulebook could provide (eg DMG2 suggests, without being entirely clear about the balancing issues, that using an attack power against the sentry will grant a +2 bonus only if the power used is an Encounter power).
Do we really need rules to tell us so much about how we play?
I can't speak for anyone else but me. But speaking for myself, where the point of the rules is to achieve some sort of mechanical balance across the spread of options for each player, and the maths is not transparent just on inspection, then yes, I do want rules to help me.
Of course I can wing it - I was a Rolemaster GM for 20 years, and GMing Rolemaster involves constantly deciding on, and then adjudicating, house rules for action resolution.
But part of the reason my group has switched from Rolmeaster to 4e is because 4e is mechanically much tighter and plays much better - less handling time, less swingy overall but more dramatic becuase often
more swingy round-by-round. This is due to the mechanics. And I want more mechanics to help me achieve this in parts of the game (ie the combat/skill challenge interface) which are currently poorly developed.
A comparison: if I sat down with a pen and paper for a few hours I might be able to work out sensible success numbers for an extended contest in HeroQuest, but the book helps by having Robin Laws do the maths and write it up in a handy table - and according to the credits he's even playtested it! I don't think it's too much to ask for 4e to offer the same mechanical support.