Well, first of all, I'm B4, so I'll probably immediately get told off, but anwyays.
Secondary main here is Jhin, I have a fair/decent number of games/experience on him. (Level 5, almost 6)
I generally hold my 4th shot for harass when I can, since I can just use it any time to farm if I'm about to lose CS (or just use Q instead).
Your abilities (Q/W/E) all refresh the duration for the 'time until reload' thing, if you want to use it to hold onto your 4th shot longer. They also all reset your auto-attacks (not sure if the E does, forgot).
Auto-Q-Auto-W-Auto-Auto and Auto-Q-Auto-E-(Auto+W+Auto/W+Auto) are both possible near-instantly flowing burst combos, the latter being a bit harder. But the second of the two combos guarantees they won't be able to escape the trap, which is a fair bit of additional damage, and quite hefty in later game.
Your E traps give a small range of vision and have a decent slow, so I generally place them along gank paths (river bush, along the openings), and then along the lane bushes for vision and harass.
Your W does very good damage in mid/late game and it can kill low-health targets easily at that point, if you can land it.
Your ultimate scales off missing health and comes with a powerful slow, so it's great for picking off fleeing/low-health targets. You also shouldn't feel mandated to use all 4 shots every time. Part of the cooldown is refunded for each unused shot, so just using however many you need is better. Also, if you fire like, one shot and then immediately exit and keep fighting/send in a W root for your team/etc, it can catch people off guard.
If you're against a hard pushing lane, I like to throw traps down the lane to slow down and break up their waves as well, since he's not too good at farming under turret with his slow AS + static 2.5 second reload time.
The crit build is far better than the Blades build, because Essence Reaver + Infinity Edge is the strongest two item spike for Jhin, possibly one of any in the whole game. GB+DB is a cheaper two item power-level, but you might as well just hold out and get the former option, because just the raw AD of B.F. Sword or the other sub-parts isn't bad at all either.
Masteries: http://puu.sh/qtoWU/f42e4e9bcc.jpg
Runes:
9x ArmorPen or flat AD marks (your choice, I prefer 3x ArmorPen/6x AD)
9x Armor Seals
9x MR Glyphs
3x AD Quints
