Fixed link.
In 4E, the easiest implementation would be to apply the rule for cover not only for enemies but for allies as well. That is, you get -2 to the attack (which is a mere -10%; remember many other rpgs would impose a -25% penalty under the same circumstances).
The difference between getting +1 for being closest and getting -2 for having your Defender stand in the way should be more than sufficient motivation for the ranged Ranger to move about. Effectively, seeking "clear shots" would be rewarded with a +3 bonus, not only a +1 bonus any more.
I really like the general idea of this, but attack bonuses are extremely important in 4E. This impacts more than just Rangers (and I assume laser Clerics, Warlocks, and Wizards were more or less balanced before this). And it's a huge nerf for the times when they're stuck in that 15' wide hallway. Yes they'd be worse off as melee characters, but it still feels bad. They'd be taking a -2 penalty for maybe even half of their attacks. Not only that, but it'd be very easy for your allies to even grant superior cover. Hit chances are low enough as it is, and characters only hit 4 or 5 times per encounter now. Reducing it by even 1 is a big deal.
Additionally, I don't think this incentivizes Prime Shot. It will incentivize players to seek an open firing lane for the +2 bonus, but that's independent with getting closest to monsters.
A way around this would be to make Prime Shot (at close range) downgrade superior cover to just cover, downgrade cover to nothing, and give a +1 bonus if they didn't have any cover to begin with. Problem is, there's still that nerf to ranged attackers that don't have Prime Shot.
In my mind, the ranged Ranger should do less damage than either the melee Ranger or the Rogue. That is, ranged Strikers should do less damage than melee Strikers. Melee strikers expose themselves to harm and needs to be compensated for that, or nobody will want to play a melee Striker.
I wholeheartedly agree, which is why I was thinking to limit the range on Hunter's Quarry. Maybe Hunter's Quarry should be limited to Range 10? Range 10 is still within move-charge range, while range 20 probably isn't.
Hmm... brainstorming here... How about, if you use prime shot, you can also designate/curse one of the attacked targets as part of the attack action?
I don't think that's a large enough bonus

. It's only an extra minor action, and the target flexibility isn't a huge deal and provides less incentive to move around.