As long as the tap and line remain chilled there isn't a problem with staling or cleanliness. And I've never had a cobra tap leak on it's own accord- always something I did like squishing it between the door and side of keg or something along those lines.
A 5' line is a little better for maintaining the carbonation in the pour. There's a formula out there to figure out the serving line lengths needed for the desired carbonation volumes.
I've heard that about 5-foot lines. Looking at various online homebrew stores, most sell 5-foot lines. There must be a reason for that. Though per online calculators from reputable sources, the length should be shorter.
L = (keg_pressure – 1 psi) / Resistance
(per
http://beersmith.com/blog/2011/07/14/keg-line-length-balancing-the-science-of-draft-beer/ )
Fridge = 38f ; Desired carbonation 2.0 (I see a lot of recipes list 2.4 for oatmeal stout, but that feels really high)
Elevation of line = 0
Beer line characteristics: 3/16" D; cold
PSI: oh wait... I have it at 10... per this it should be at 6:
https://www.homebrewersassociation.org/attachments/0000/1312/Summerzym95-Kegging_How-To.pdfSo I ended up carbonating at 2.4 anyway! I can't remember why I set it at 10. Though I have to say I have the taste impression that level of carbonation took what was a mediocre (for me) stout that finished too sweet (aiming for more mouthfeel, I think I mashed too high) and made it much better.
But anyway, following the formula, the line should be 3 feet long: (10 – 1) / 3. If I went down to 8, then it's L=(8-1)/3 which is even shorter; if I went down to 6, shorter yet. But I have yet to hear of anyone using that short of a line.
So in conclusion... I'll ignore the formulas, keep using the four-foot line, and when it's time to build a second line, make it five feet long. :-)